Unlike most stuffed mushroom recipes, these savory sausage-filled gems don't contain cheese, so they're a great choice for people avoiding dairy. The recipe comes from Annie Marshall, of the Indianapolis-based blog Annie's Eats.
Clean the mushroom caps with a damp paper towel. Remove the stems from the caps. Coarsely chop the stems and set aside.
Crumble sausage into a large skillet and cook over medium-high heat, stirring occasionally, until no longer pink. Stir in the chopped mushroom stems and continue to cook until they have released most of their juices, about 5 minutes. Stir in the garlic and crushed red pepper, if desired, and cook just until fragrant, about 30 seconds more. Stir in the spinach; cover and cook just until spinach is wilted, about 2 minutes. Transfer sausage mixture to a food processor or blender. Cover and pulse until mixture is evenly chopped. Add the egg and pulse just until incorporated.
Fill each mushroom cap with about 1 tablespoon of the sausage mixture. Place the filled caps on a foil-lined 15x10x1-inch baking pan. In a small bowl stir together the panko, dried basil, salt and pepper. Add olive oil and toss to combine. Sprinkle a bit of the panko mixture over the top of each stuffed mushroom. Bake in a 400°F oven for 18 to 20 minutes or until topping is golden. Top each with additional spinach, if desired.