Melt the fat in a pot. Stir in flour, sweat briefly. Add milk and stock little by little while stirring. Bring to the boil, simmer for 6-8 minutes, remove from heat. Crumble 100 g cheese into the sauce and melt while stirring.
Season to taste with salt, pepper, nutmeg and lemon juice. Remove from heat.
Peel onions and garlic and cut into small pieces. Clean and wash the spinach and let it drain in a sieve. Heat olive oil in a tall, wide pot. Fry the onions and garlic for 2-3 minutes, turning them over.
Add the spinach, cover and let it collapse for 2-3 minutes. Season to taste with salt, pepper and nutmeg.
Peel and wash the potatoes and slice them finely. Layer a thin layer of potato slices in an ovenproof casserole dish (19 x 27 cm; 6 cm high). Season with salt and pepper. Spread the spinach on top and pour on some béchamel sauce.
Spread the remaining potatoes on top and pour the rest of the sauce over it. Sprinkle with 100 g cheese and bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 60-70 minutes.
Remove and serve immediately.