Preheat oven to 400°F (200°C). Wipe the portobello caps with a damp paper towel to remove dirt. Remove and discard stems and gently scrape out the dark gills with a spoon to create a cavity for stuffing (optional but reduces moisture).

Place mushroom caps gill side up on a rimmed baking sheet lined with parchment. Brush each cap with 1/2 tablespoon olive oil and season inside and out with about 1/4 teaspoon salt and a light sprinkle of black pepper. Set aside while you make the filling.

Filling

Cook aromatics

Heat 1 tablespoon olive oil and 1 tablespoon butter in a medium skillet over medium heat until butter foams. Add the finely chopped onion and a pinch of salt; sauté 5–6 minutes until translucent and starting to brown at the edges. Add the diced red bell pepper and cook 2–3 minutes until softened. Add minced garlic and cook 30–45 seconds until fragrant, taking care not to burn.

Wilt spinach & combine

Add the chopped spinach to the skillet and cook, stirring, until wilted (about 2 minutes). Remove the skillet from the heat and allow the mixture to cool for 2–3 minutes.

Mix filling ingredients

Transfer the sautéed vegetables to a mixing bowl. Stir in the crumbled goat cheese, grated Parmesan, panko breadcrumbs, chopped parsley, thyme, dried oregano, red pepper flakes, lemon zest and lemon juice. Season with 3/4 teaspoon salt and 1/2 teaspoon black pepper. If you prefer a more cohesive filling, stir in the beaten egg now to bind; otherwise omit for a looser texture. Taste and adjust seasoning.

Stuffing & Baking

Fill caps

Divide the filling evenly among the 4 prepared mushroom caps, mounding it slightly so each is generously filled.

Top (optional)

If using, sprinkle about 2 tablespoons additional panko across the 4 caps and then the shredded mozzarella. Drizzle the panko lightly with 1 teaspoon olive oil for extra crispness.

Bake

Bake in the preheated 400°F (200°C) oven for 18–22 minutes, until mushrooms are tender and filling is heated through and top is golden. If you want a browner cheese top, switch oven to broil for the last 1–2 minutes—watch closely to prevent burning.

Remove from oven and let rest 3–5 minutes. Sprinkle with additional chopped parsley and a tiny extra squeeze of lemon if desired before serving.