For the pancakes, mix all ingredients until smooth. Leave to swell for about 1/2 hour. Heat a little butter in a coated frying pan.
Pour in a thin layer of batter and bake until golden on both sides. Remove from the frying pan and cool on a cooling rack.
For the filling, remove the stems from the spinach leaves, rinse, blanch in salted water, rinse in iced water and squeeze.
Sauté finely chopped onions and garlic in olive oil until colorless. Add all remaining ingredients and season with salt, pepper and a pinch of nutmeg.
Lay out the cold pancakes, spread with the spinach filling, roll up and place in a greased gratin dish.
Mix the milk with the eggs and pour over the pancakes. Sprinkle with grated cheese and bake in the oven at 200 °C for about 20 minutes.