Preheat the oven to 375ºF.
Remove the stems from the mushrooms, set aside to dice and add to the filling. Brush mushrooms with a little olive oil, place stem side up and bake for 8-10 minutes.
In a medium pan over medium-high heat, add 1 tablespoon of olive oil. Saute the shallots, bell pepper, garlic and sun dried tomatoes for about 3 minutes until they start to get tender. Add in the parsley, finely chopped mushroom stems and chopped walnuts, cook another 2-3 minutes. Add in chopped baby spinach and allow it to wilt. Salt and pepper to taste.
Remove the mushrooms from the oven, pour off or soak up any water that accumulates in the mushrooms. Spoon the filling into the mushrooms and bake for an additional 10-12 minutes. Topped with fresh, raw diced red bell peppers and serve warm.