Drain your ricotta now: place it in a fine-mesh sieve over a bowl for 15 minutes (or wrap in cheesecloth and squeeze). This single step makes the difference between creamy and watery filling.

Preheat oven to 400°F (205°C) and line a large baking sheet with parchment. Remove stems from mushrooms and scrape out every bit of the dark gills with a teaspoon (this prevents grey, bitter filling).

Lightly brush caps all over with 2 tbsp olive oil, place gill-side DOWN on the sheet, and bake exactly 10 minutes. You’ll see liquid pool underneath — perfect!

While they bake, make the spinach: heat remaining 1 tbsp oil/butter in a large skillet. Add minced garlic for 30 seconds until fragrant, then add all the spinach. Sauté 2–3 minutes until completely wilted, then keep cooking another 2 minutes until the pan is dry. Spread on a cutting board to cool, then chop finely.

In a large bowl, combine drained ricotta, chopped spinach, egg yolk, ½ cup Parmesan, lemon zest + juice, nutmeg, salt, and pepper. Mix vigorously until silky smooth. Taste — it should make you say “wow.” Adjust seasoning.

Remove mushrooms from oven. Flip them over, pour off all liquid, and pat insides completely dry with paper towels. Your mushroom cups are now ready!

Place a few small pieces of mozzarella (about 1 teaspoon total) in the bottom of each cap — these melt into hidden cheese surprises.

Spoon a generous 1½ tablespoons of spinach-ricotta filling into each mushroom, pressing gently and mounding high. Really pack it in — they shrink a bit.

Top each with remaining torn mozzarella and a sprinkle of the reserved ¼ cup Parmesan. Bake 12–14 minutes until mushrooms are tender and cheese is melted, then switch to BROIL (high) for 90 seconds until golden and blistered in spots — watch constantly!

Rest exactly 8 minutes (they’re lava-hot inside). Transfer to a platter, grate a little extra Parmesan over the top, scatter chopped parsley and a pinch of red pepper flakes if desired, and serve warm. Watch them vanish in seconds.