Prep. Heat oven to 400F. Line a sheet pan with parchment.
Clean and stem. Wipe mushrooms clean. Remove stems and finely chop them.
Sauté filling base. Heat olive oil and butter in a skillet over medium heat. Add chopped mushroom stems and onion. Cook 4 to 5 minutes until softened and most moisture evaporates. Add garlic and cook 30 seconds. Turn off heat.
Mix filling. In a bowl, combine cream cheese, Parmesan, breadcrumbs, parsley, salt, pepper, red pepper flakes, and lemon juice (if using). Stir in the sautéed mixture.
Stuff. Arrange mushroom caps on the sheet pan. Spoon filling into each cap, mounding slightly. Sprinkle with extra Parmesan if you like.
Bake. Bake 16 to 20 minutes until mushrooms are tender and tops are golden. Optional: broil 1 to 2 minutes for extra browning.
Serve. Serve warm.