Prepare mushrooms by snapping off stems and removing gills with a teaspoon, being careful not to break mushroom. Roughly chop stems and reserve for filling.
Place mushrooms on a baking sheet, cavity side up, covered with a damp paper towel until ready to use.
In a large non-stick skillet, sauté onion until soft. Add garlic and continue to sauté for 30 seconds, or until fragrant.
Add chopped mushroom stems and a pinch of salt. Cook until tender.
Add spinach and sauté until wilted. Stir in tomatoes, pine nuts, oregano, balsamic vinegar and pepper (to taste.)
Stir in oats, remove from heat and let stand for 5 minutes before stuffing mushrooms.
Stuff mushroom caps with 1 heaping tablespoon of the filling. Sprinkle each with parmesan cheese.
Bake at 375° F (190° C) for approximately 12-15 minutes, or until cheese is golden and mushrooms are tender. Transfer to a serving plate and sprinkle with parsley.