Prep the mushrooms: Heat the oven to 400°F (200°C). Wipe the mushrooms clean with a damp paper towel. Twist out the stems and finely chop them. Brush the mushroom caps with 1 tablespoon of olive oil and place them hollow-side up on a baking sheet.

Toast the walnuts: In a dry skillet over medium heat, toast the walnuts for 2–3 minutes, stirring, until fragrant. Transfer to a cutting board, let cool, and chop finely.

Sauté the aromatics: In the same skillet, add the remaining 1 tablespoon of olive oil. Cook the onion and chopped mushroom stems with a pinch of salt for 4–5 minutes, until softened and lightly golden.

Add garlic and spinach: Stir in the garlic and cook 30 seconds. Add spinach and cook until wilted (or warmed through if using thawed). Season with salt, pepper, and red pepper flakes if using.

Build the filling: Take the skillet off the heat. Stir in walnuts, Parmesan, breadcrumbs, and parsley. Taste and adjust seasoning. The mixture should hold together and feel slightly tacky; add a touch more breadcrumbs if too wet.

Stuff the caps: Spoon the filling into each mushroom, gently mounding it. Sprinkle a little extra Parmesan on top if you like.

Bake: Bake for 15–18 minutes, until the mushrooms are tender and the tops are lightly golden. If you want more color, broil for 1–2 minutes at the end.

Serve: Let cool for 5 minutes so the filling sets. Finish with a crack of black pepper and a drizzle of olive oil. Serve warm.