Prep the mushrooms: Wipe the mushroom caps with a damp paper towel. Twist out the stems and save them. Use a small spoon to gently hollow the caps if needed. Arrange the caps on a parchment-lined baking sheet, cavity side up. Lightly brush with olive oil and season with salt and pepper.

Chop the stems: Finely mince the mushroom stems. This adds umami to your filling and reduces waste.

Sauté the aromatics: Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the onion and cook until soft and translucent, about 3–4 minutes. Stir in the minced mushroom stems and cook another 2 minutes. Add the garlic and cook until fragrant, about 30 seconds.

Cook the beef: Add the ground beef to the skillet. Season with 1 teaspoon salt and 1/2 teaspoon black pepper. Cook, breaking it up, until browned and no longer pink, about 5–7 minutes. If there’s excess fat, drain it.

Wilt the spinach: Add the spinach to the skillet in batches. Stir until wilted and most moisture evaporates, 2–3 minutes. You want the mixture fairly dry so the mushrooms don’t turn soggy.

Fold in the cheeses: Remove the skillet from heat. Stir in the cream cheese until melted and creamy. Add the Parmesan, bread crumbs, and parsley. If using red pepper flakes, add them here. Taste and adjust seasoning.

Stuff the caps: Spoon a heaping mound of filling into each mushroom cap, pressing gently so it stays in place. The filling should be slightly domed.

Bake: Bake at 375°F (190°C) for 18–22 minutes, until the mushrooms are tender and the tops are lightly golden. For extra color, broil for 1–2 minutes at the end.

Finish and serve: Let the mushrooms rest for 5 minutes so the filling sets. Add a light squeeze of lemon and a sprinkle of parsley if you like. Serve warm.