Preheat oven to 400°F. In a medium skillet, sauté pancetta over medium-high heat until crisp, about 3 minutes. Remove to a mixing bowl. Discard fat from skillet. Melt butter in same skillet over medium heat. Add onion and garlic; sauté 3 minutes. Stir in spinach; cook just until wilted. Add to pancetta; allow to cool slightly. Stir in cheese, bread crumbs and basil. Season with salt and pepper.
Place mushrooms on baking sheet. Mound about 2 1/2 tablespoons cheese mixture into center of each mushroom cap. Bake for 8 minutes. Serve hot.