Prep the mushrooms: Preheat your oven to 400°F (200°C). Line a rimmed baking sheet with parchment. Wipe mushrooms clean with a damp towel and gently twist out the stems. Finely chop the stems and set aside. Lightly brush the mushroom caps with olive oil and place them cavity-side up on the baking sheet.

Cook the sausage: Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the sausage, breaking it up with a spatula, and cook until browned and no longer pink, about 5–7 minutes. Drain excess grease if necessary.

Sauté aromatics: Add the chopped mushroom stems and minced garlic to the skillet. Cook until the stems release moisture and soften, 2–3 minutes. Season with a small pinch of salt and pepper.

Finish the filling: Reduce the heat to low. Stir in cream cheese until fully melted and smooth. Add 1/4 cup Parmesan, mozzarella, parsley, onion powder, and red pepper flakes if using. Taste and adjust seasoning with salt, pepper, and a hint of lemon zest if desired.

Stuff the caps: Spoon or pipe the warm filling into each mushroom cap, rounding slightly on top. Sprinkle the remaining 1/4 cup Parmesan over the filled mushrooms for a golden finish.

Bake: Bake for 15–18 minutes, or until the mushrooms are tender, the tops are lightly browned, and the filling is bubbling at the edges.

Rest and serve: Let the mushrooms rest for 5 minutes to set. Garnish with a little fresh parsley. Serve warm.