Easy, cheesy, beautiful stuffed mushrooms make for the the perfect vegetarian party appetizer.
(See notes for non-vegetarian option.)
1. Preheat the oven to 350°F, with the rack in the middle.
2. Gently clean the mushrooms with a damp paper towel or kitchen towel to remove any dirt.
3. Remove the stems by holding the cap in your hand and gently wiggling the stem until it releases. Chop the mushroom stems extremely fine, discarding the tough ends, and set aside.
4. Use a spoon to gently scrape the gills out of the mushroom caps to make room for the stuffing.
5. Place the mushroom caps in a large bowl and gently toss with ½ tbsp olive oil, ⅛ tsp salt, and ⅛ tsp black pepper, making sure not to break the mushrooms. Arrange the mushroom caps stem-side up on a and set aside.
6. Heat the remaining 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering, then add the celery and shallot and cook until tender, about 3 minutes.
7. Add the chopped mushroom stems, minced garlic, cayenne pepper (if using), the remaining ⅛ tsp salt, and the ⅛ tsp black pepper.
8. Cook until the liquid evaporates and the mushrooms begin to brown, taking care not to burn the garlic, about 10 minutes.
Add ⅛ cup white wine or vegetable broth to deglaze the pan. Cook until the liquid evaporates, about 1 minute.
10. Remove the pan from the heat and stir in the chopped parsley and panko breadcrumbs.
11. In a large bowl, whisk together the mozzarella cheese, Parmesan cheese, and egg.
12. Add the breadcrumb mixture and stir to combine.
13. * Divide the filling between the mushroom caps and stuff the mushrooms, about 2 tsp each for 2-inch wide mushrooms or 1 tsp for 1-inch wide mushrooms. The filling may mound up, but press it down to fill the entire cavity.
14. Bake the mushrooms on the rimmed baking sheet until tender, the filling is hot, and the tops are golden brown, about 20 minutes. For some extra crisp on top, broil for ab out 2 to 5 minutes.