This crowd pleaser will be a favorite as an appetizer or first course.
Preheat oven to 375°F.
Clean mushrooms by wiping with damp cloth. Carefully break off stems and set the caps aside. Finely chop stems, discarding any fibrous or tough ends.
Brush a large baking dish with 1 tablespoon olive oil.
Heat remaining 2 tablespoons olive oil and butter in a large sauté pan. Add the onions and cook, stirring occasionally, for 4 minutes. Add the garlic and cook for 30 seconds. Add the chopped mushroom stems, parsley and pepper and continue to cook, stirring frequently, for 3 minutes. Add chicken stock or broth, bring to a simmer and stir in the baby spinach.
When greens are wilted, remove pan from heat and stir in bread crumbs and Gorgonzola cheese, blending well.
When mixture is cooled enough to handle, using a small spoon, stuff each mushroom cap generously with the spinach mixture. Top with a sprinkle of Parmesan cheese.
Arrange stuffed mushrooms in oiled baking dish and bake for 15 to 20 minutes. The mushrooms are ready when firm but cooked throughout.