Prepare the mushrooms by wiping off the caps and removing all of the stems.
Line the mushrooms up in a baking dish, cap side down. Scoop approximately 1/2 teaspoon of marinara sauce into the bottom of each cap. You may need a little more or less depending on how large they are. Use just enough to cover most of the bottom of the hole.
Scoop 1 teaspoon of ricotta cheese into each mushroom cap. Gently push it down to fill the hole. It should be just below level with the top of the mushroom.
Scoop another 1/2 teaspoon of marinara on top of the ricotta. It is okay if it bulges up above the top of the cap.
Sprinkle each mushroom with the mozzarella cheese and Parmesan cheese. Then sprinkle with dried oregano.
Bake the mushrooms for 10-15 minutes until the mushrooms are tender. Bake longer if you want softer mushrooms, however they will become more difficult to eat by hand. Serve immediately.
Notes
If you want your mushrooms browned on top but it is not happening, pop them under the broiler in the oven for just a minute or two. Make sure they are NOT on a silicone baking mat if you do this as it will blister and burn. Just put them on a metal pan or in a baking dish.You may need more or less of the sauce and cheeses depending on exactly how large the mushrooms are. Fill with each ingredient to your desired preference.To ensure keto, use a keto marinara.