Remove the stems from the mushroom, chop stems and set aside. Wipe the mushroom caps clean with a damp paper towel.
Heat the butter in a large skillet over medium heat; add the chopped mushroom stems, onion, and garlic and cook for 6 to 8 minutes, or until tender. Add chopped shrimp and cook for 2-3 minutes or until shrimp are just pink; add breadcrumbs, beaten egg, salt, lemon juice, basil, and the pinch of red pepper flakes. Cook for 2 to 3 minutes, stirring as needed.
Place the mushroom caps, stem side up on a baking sheet. Divide the shrimp mixture among the caps and sprinkle with parmigiano-reggiano.
Bake mushrooms for 20 minutes. Sprinkle with chopped parsley before serving.
Notes
Recipe adapted from Little Spice Jar and originally, Louisiana Seafood