Brush the potatoes under running water. Cook in boiling water for about 15 minutes. In the meantime heat up fat in a pot, stir in flour and sweat on for about 5 minutes at low heat.
Stir in the stock and milk and simmer at low heat for about 10 minutes. Wash lemon with hot water, grate peel and squeeze 1/2 lemon. Season béchamel with salt, nutmeg and lemon juice.
In the meantime clean, wash and drain the spinach. Dip briefly in boiling salted water, rinse in cold water and drain. Wash, clean and halve the cherry tomatoes. Drain the potatoes, let them evaporate briefly and cut in half.
Place the potatoes with spinach and cherry tomatoes in an ovenproof casserole dish with lid. Season with salt and pepper. Pour over the béchamel. Finely grate the Parmesan cheese. Spread the parmesan with lemon zest and pine nuts on the casserole dish and cover with the lid.
Bake in the preheated oven (electric: 225 ° C/ gas: level 4) for about 40 minutes.