Let's pre-cook the mushrooms. This will shorten the cooking time.
Set oven to 350ยบ F
Place the cleaned mushrooms on an oiled baking sheet and put in the oven for 20 minutes.
While the Mushrooms are in the oven lets make the sauce:
Add 2 tbsp of Oil to a medium sized saucepan on medium heat. Let the oil heat up. Add the Onions and Garlic to the pan and saute for about 5 minutes or until onions are translucent.
Add Diced Tomatoes, Tomato Sauce, Salt, Pepper, & Italian Seasoning to pan. Stir and bring to a boil about 5-10 minutes. Once it reaches a boil to turn off the heat.
Now to constructing the mushrooms:
Remove the mushrooms from the oven. You will notice liquid pooling in the center of the mushrooms. This is from the mushroom releasing its water content. Remove each mushroom and pour out the liquid into the sink. Place back on the baking sheet.
Add a 1/4 cup of Ricotta to each mushroom. Layer Sauce and Mozzarella Cheese on top.
Place back in the oven for another 20 minutes or until Mozzarella Cheese is melted.