Mushroom Onion Casserole

Mushroom Onion Casserole

I’m not a huge fan of the word ‘casserole.’ It always brings to my mind something a little gloopy that’s made mostly with boxes and cans and almost no fresh ingredients. Those things can be surprisingly delicious (a friend brought an excellent green bean casserole to Thanksgiving last year) but I’m often a little skeeved out by what’s on the label. Yes, I’m a bit of a snob when it comes to foods that come in a package and shouldn’t have as long a shelf life as they do. It’s not that I’ll never eat them but I would be lying if I said that I make them in my kitchen. Which is why I struggled about calling this a Mushroom Onion casserole.

I was worried that you, like me, might be a bit of a snob when it comes to food and see the word ‘casserole’ and think it’s probably not something you’d want to make. Because you want to make this. It’s creamy and rich and kind of amazing considering how few ingredients are in it.

I thought about dressing up the title, adding ‘caramelized’ to describe the onions and mushrooms and talking up the ‘walnut crumble’ on top. But it felt kind of weird. This isn’t a fancy cooking dish. It has no delicate crust and it doesn’t make clean slices. It’s warm and comforting and it makes me want to curl up in a thick sweater and slippers and eat a big bowlful. It is truly a casserole – baked in a casserole dish creamy ingredients that bind it together, without a crust to make it a pie. I’m ok with that if you are.

With three kinds of cheese to its name, this isn’t winning any awards for being low-fat. But its richness also comes from the deeply caramelized vegetables that make up the bulk of the dish. The sharpness of the feta is a welcome contrast to all that creamy softness.

I could imagine all sorts of ways to dress this up – use fancier mushrooms, add some additional herbs (like thyme and oregano or add a kick with some chile flakes), caramelize the onions slowly, or add a more luxurious cheese to make this a dish that is all about richness. But I also love that it comes together with just the things you find in the grocery store. Not the fancy gourmet grocery store. The regular one. Where you can buy the things for those other casseroles. You know what I mean.

In case you haven’t checked the calendar, it’s November which means it’s NaBloPoMo. This month I’ll be trying to post a new tidbit every day. Let’s spend it together, shall we? In the meantime, if you have things you’d like to see, we have plenty of time so leave a comment below and I’ll try to get to it.

Mushroom Onion Casserole


  • 1 ½ lb mushrooms finely sliced
  • 3 onions finely sliced in half-moons
  • 4 tbs fat*
  • 3 tbs salt
  • 1/2 cup/ 4 oz ricotta
  • ½ cup feta
  • 1 egg
  • ½ cup parm
  • 1 ½ cups chopped walnuts
  • ½ cup finely chopped parsley or one medium bunch**
  • Freshly ground pepper


  1. Heat a large skillet over medium heat and add a tablespoon of fat. When it’s warmed add the onions and about a teaspoon of kosher salt and cook the onions until they caramelize. If you’re using the method discussed by me few days back for quicker caramelization, use the scraping method twice and add 1 tbs of Worcestershire sauce for flavor. Traditional caramelization could take an hour or two and it’s not really necessary since onions are just part of this dish.
  2. When the onions are done remove them to a bowl, wipe out the skillet, return it to medium heat, and add another tablespoon of fat.
  3. Add the mushrooms in relatively small batches to avoid crowding the pan – mine made about 6 batches total. Cook the first batch until the mushrooms have lost some water and have started to brown on at least one side, about five minutes. Then push that batch to the outside edges of the pan and add a new batch to the middle. For each batch after the first take a minute to stir the older and newer mushrooms together when the middle newer mushrooms are partly cooked enough to push them out to the edges. Add the remaining fat, one tablespoon at a time when the pan looks too dry and the mushrooms on the edges start to stick. When you do, make sure to swirl it around the pan so that it goes all the way out to those edge mushrooms.
  4. Cook the mushrooms until all of them are at least lightly browned and they have lost a noticeable amount of water weight. Depending on the number of batches you do this could take 30-45 minutes. When the mushrooms are browned to your liking, add a teaspoon or so of salt to season them.***
  5. When the mushrooms are done, turn the oven to 350F. Grease an 8 by 11 casserole dish (a typical 9 by 13 pan will be too big) and set it aside.
  6. In a large mixing bowl combine the onions and mushrooms and set them aside for 20 minutes to cool. When it’s cooled add the egg, ricotta, feta, 2 teaspoons of salt, and as much pepper as you like.
  7. Spread the mixture into the greased casserole dish and flatten the top using a spatula. Combine the chopped walnuts with the parmesan and sprinkle that over the top.
  8. Bake the casserole at 350 for 30 minutes, then turn on the broiler and broil it for 2-3 minutes just to toast the top. Keep an eye on it so the walnuts don’t burn. Sprinkle the finished dish with parsley right before you serve it.

Recipe Notes

In casserole dishes like this, use whatever kind of fat you like. Butter will add some extra richness. Olive oil will add a pleasant bitterness. We have a fairly constant stock of rendered animal fat so I use that.

Yes, the parsley adds a much-appreciated bit of color (we do eat with our eyes too after all) but don’t think that it doesn’t also serve a flavor function. The fresh greenness of it goes well with the creamy richness of the casserole. You’ll notice if it’s not there.

Typically when I cook things I season everything while I cook it. But mushrooms have a lot of water and salt draws it out. Adding it at the end means those leftover juices can be mixed into the filling rather than being evaporated in the pan.

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