Preheat oven to 400°F.
Over medium heat, heat the oil in the dutch oven.
Add your diced onion and cook about five minutes, until translucent.
Add the garlic and cook about a minute.
Add the tomatoes and mushrooms, and simmer for about 10 minutes.
While simmering, mix the ricotta, cottage cheese, egg, oregano, salt, and spinach in a mixing bowl. Set aside.
In another bowl, mix the mozzarella and Parmesan together.
Transfer the sauce to another mixing bowl, leaving the dutch oven empty.
Assemble your lasagna layers. Add a layer of sauce, then of noodles, then ricotta mix, then mozzarella mix, then repeat. Sauce -> Noodles -> Ricotta -> Mozzarella. You should have enough ingredients for 3-4 layers depending on the size of your dutch oven.
Top with any additional mozzarella and parmesan. Bake uncovered for 40-45 minutes, or until the noodles have your preferred texture.
Let cool for about 10 minutes out of the oven before digging in and serving. Enjoy!