Prep the mushrooms: Heat your oven to 400°F (200°C). Line a sheet pan with parchment. Gently remove the mushroom stems, scrape out the dark gills with a spoon, and wipe the caps clean. Brush both sides with olive oil and season with salt and pepper.

Pre-roast the caps: Place the caps gill-side up on the sheet pan. Roast for 8–10 minutes to soften and release moisture. Carefully blot any pooled liquid with a paper towel and set the caps aside.

Cook the aromatics: In a skillet over medium heat, warm 2 tablespoons olive oil. Add the onion and a pinch of salt. Cook 3–4 minutes until translucent, then stir in the garlic for 30 seconds until fragrant.

Add the spinach: Pile in the spinach. Cook, stirring, until wilted and most moisture cooks off, about 2–3 minutes. Stir in the sun-dried tomatoes, Italian seasoning, red pepper flakes, and balsamic vinegar. Cook 1 minute to reduce slightly. Remove from heat.

Make the filling: Stir in the breadcrumbs, Parmesan, and half of the mozzarella. Taste and adjust salt and pepper. The mixture should be moist and cohesive, not wet. Add a drizzle of olive oil if it seems dry.

Stuff the caps: Spoon the filling evenly into the pre-roasted mushrooms, mounding it slightly. Top with the remaining mozzarella.

Bake: Return the stuffed mushrooms to the oven and bake 10–12 minutes, until the cheese is melted and golden in spots and the mushrooms are tender.

Finish and serve: Let them rest 3 minutes. Sprinkle with chopped parsley and a squeeze of lemon if you like. Serve hot with a simple green salad or toasted bread.