Prep the mushrooms. Heat the oven to 400°F (200°C). Wipe the portobellos clean with a damp paper towel. Twist off any stems and gently scrape out very dark gills with a spoon if you prefer a milder flavor. Brush both sides with olive oil and season with salt and pepper.

Pre-bake the caps. Place mushrooms gill-side up on a foil- or parchment-lined sheet. Bake for 10–12 minutes until they release some liquid and start to soften. Carefully drain off any pooled liquid and pat the insides dry with a paper towel.

Cook the spinach. While the mushrooms bake, heat a drizzle of olive oil in a skillet over medium. Add garlic and cook 30 seconds until fragrant. Add fresh spinach and a pinch of salt; sauté until wilted and most moisture evaporates, about 3–4 minutes. If using frozen spinach, just warm it with the garlic and cook off excess water. Let it cool slightly, then chop and squeeze out any extra liquid.

Mix the filling. In a bowl, combine cream cheese, mozzarella, Parmesan, chopped spinach, lemon zest, 1–2 teaspoons lemon juice, red pepper flakes if using, and a few grinds of black pepper. Taste and adjust salt, pepper, and lemon to your liking.

Fill the mushrooms. Divide the spinach-cheese mixture among the pre-baked caps. Mound it slightly and smooth the tops. For crunch, mix breadcrumbs with a teaspoon of olive oil and a tablespoon of Parmesan, then sprinkle over the filling.

Bake until bubbly. Return the sheet to the oven and bake 12–15 minutes, until the filling is hot and the tops are golden. If you want extra browning, broil for 1–2 minutes at the end. Watch closely.

Finish and serve. Let the mushrooms rest 3–5 minutes so the filling sets. Sprinkle with chopped parsley or chives and an extra squeeze of lemon if you like. Serve warm with salad, grains, or toasted bread.