Peel the potatoes, wash them thoroughly and cut or slice them into thin slices. Grease a tart mould (20 cm Ø) and line it fan-shaped with the potato slices, seasoning each layer with a little salt. Melt butter and pour over the potatoes. Bake the potatoes in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 40 minutes
Cut the roulades in half, beat flatter between 2 layers of foil. Drain mozzarella and cut into slices. Wash the spinach and drain well. Season roulades with salt and pepper. Cover with spinach and mozzarella and roll up. Pin with wooden skewers
Peel and finely chop the onion. Heat the oil in a frying pan. Fry the roulades for 3-4 minutes while turning. Take out. Sauté onion and tomato paste in the pot. Add bay leaf, tomato pieces and stock, season with salt and pepper. Add roulades. Cover and stew for about 20 minutes.
Season the tomato sauce to taste again. Remove potatoes from the oven and cut into quarters. Arrange 2 roulades with sauce and a quarter of a potato tart on plates and serve immediately