Peel and cook the potatoes for 20 minutes, drain and mash. Add semolina, egg and nutmeg and mix until it forms a ball. Dust a surface with semolina flour, divide mixture into four and roll into a sausage just over 2.5cm (1in) diameter. Cut into 2.5cm (1in) lengths, making little pillows of gnocchi.
Add gnocchi to pan of boiling water, cook for 2 minutes and remove with a slotted spoon.
Heat oven to Gas 4, 180°C, fan 160°C. Cook sliced mushrooms in a pan with a little oil for 5 minutes, add garlic and cook for 2 minutes. In a bowl mix the gnocchi, spinach and mushrooms with half the grated cheese and the crème fraîche, place in an ovenproof dish, top with rest of the grated cheese and bake in the top of the oven for 15-20 minutes.