Brush the mushrooms, turn out the stems and scrape out the lamellae with a teaspoon. Sauté the finely diced onion with the diced mushroom stems in olive oil and then deglaze with lemon juice.
Now add the coarsely chopped spinach and the crumbled feta cheese and heat briefly.
Before stuffing the mixture into the mushrooms, season with salt and pepper, or garlic if you like.
Place the mushrooms in a greased ovenproof dish, cover with butter and breadcrumbs and bake at 180 degrees (preheated) for 20 minutes until golden brown. Serve with a salad with vinaigrette and baguette.