Preheat oven to 200°C (400°F). Clean the portobello caps with a damp paper towel. Remove stems and scrape out the dark gills with a spoon to create a cavity for the filling. Lightly brush both sides of each cap with 1 tbs olive oil and a pinch of salt and pepper.

Sauté aromatics

In a large skillet over medium heat, melt 2 tbs unsalted butter. Add the finely chopped onion and a pinch of salt. Cook, stirring occasionally, until the onion is soft and translucent, 6–8 minutes. Add the minced garlic and cook 30–45 seconds until fragrant.

Cook spinach and season

Add the chopped spinach (it will wilt significantly). Stir until wilted and any excess liquid has mostly evaporated, about 2–3 minutes. Stir in the fresh thyme leaves, Worcestershire sauce, 1 tsp salt, and 1 tsp black pepper. Taste and adjust seasoning. If the pan is dry, splash in 1–2 tbs of the white wine to deglaze and concentrate flavors.

Assemble filling into caps

Spoon an even amount of the spinach-onion mixture into the cavity of each prepared portobello cap, pressing gently so the filling sits snugly in the mushroom.

Cheese & Topping

Add Swiss and topping

Lay one Swiss cheese slice over the filled top of each mushroom (trim slices if necessary to fit). Sprinkle the grated Parmesan evenly over the tops, then, if using, sprinkle panko breadcrumbs for extra crunch.

Bake

Arrange the filled mushrooms on a rimmed baking sheet or in an oven-proof skillet. Pour the remaining white wine (or vegetable broth) into the pan around the mushrooms (not over them) to keep them moist. Bake in the preheated oven for 12–15 minutes, until cheese is melted, bubbling and lightly golden and mushrooms are tender but still hold their shape. For extra browning, switch to broil for the final 1–2 minutes—watch closely to prevent burning.

Finishing & Serving

Remove mushrooms from oven. Drizzle lemon juice over the tops if using, sprinkle with chopped fresh parsley and a pinch of red pepper flakes for heat if desired. Let rest 2–3 minutes, then serve warm.