Preheat Oven: Preheat your oven to 450°F (232°C).
Remove stems and gills from the mushrooms, brush with olive oil, sprinkle with salt and place on a baking sheet.
For added flavor, you can also make a basic balsamic vinaigrette with the olive oil, a spoon full of balsamic vinegar and a sprinkle of salt.
Rub the mushrooms with the vinaigrette and let sit for 15 minutes.
After mushrooms have had a chance to absorb some of the oil/vinaigrette, bake at 450°F for 15 minutes until tender.
Sauté the onion and garlic in olive oil until soft. Add the spinach and tomatoes, cooking until the spinach wilts.
In a bowl, mix the sautéed vegetables, gluten-free breadcrumbs, and cheeses. Season with salt and pepper. Distribute the filling evenly into the mushroom caps, pressing down lightly.
OR chop mushrooms, add to bowl of vegetables and cheeses, add an extra ½ cup of cheese and fill 4 ramekins.
For stuffed mushrooms: Bake for 10-15 minutes or until the mushrooms are fully tender and the cheese is golden brown.
For ramekins: Bake for 30 minutes at 375 degrees
While the mushrooms are baking, mix together the balsamic vinegar and sugar in a small sauce pan.
Bring the mixture to a boil over medium-high heat, then reduce the heat to medium-low and simmer. Stir occasionally.
Let the mixture reduce until it thickens to a syrupy consistency, about 15-20 minutes. It should coat the back of a spoon.
Drizzle the mushrooms with balsamic glaze, garnish with fresh parsley and serve hot.
