Preheat oven to 400. And heat olive oil at medium heat in a large skillet. Dice onion and add to skillet when hot.
Rough chop spinach while onions cook. When onions are translucent, add crushed garlic. Cook till fragrant.
Add chopped spinach and cook till slightly wilted but still bright green. Salt and pepper to taste.
Add mixture to mixing bowl and combine with ricotta.
Clean mushrooms and remove stems and gills.
Fill mushrooms with cheese mixture. About ¼ per mushroom, but don’t overfill them.
Top with 1 T pesto and 2 T shredded mozzarella.
Bake in oven for 15 minutes. Dice tomatoes while mushrooms cook.
Top with tomatoes and fresh basil and balsamic vinegar