Peel and finely dice the onion and garlic. Heat 1 tablespoon of butter in a saucepan. Sauté onion and garlic in it. Add spinach and 2 tablespoons of water, cover and cook over medium heat for 10-15 minutes, season with salt, pepper and nutmeg.
Meanwhile clean the mushrooms and cut the stems flat (do not break them out). Cut the bacon slices in half and leave them crispy in an ovenproof pan, take them out and drain them on kitchen paper.
Add oil to the bacon fat and fry the mushrooms lightly on both sides, season with salt and pepper. Stir sauce hollandaise powder into 125 ml cold water. Bring to the boil while stirring.
Switch off the stove. Beat the butter in pieces until it has melted. Remove from the stove. Turn the mushrooms in the pan with the stalk side up. Spread spinach on top, sprinkle with cheese and pour the hollandaise sauce over it.
Grill under the hot grill for 6-7 minutes. Arrange mushrooms with one bacon piece each on a plate.