Peel and wash the potatoes and cook them in boiling salted water for 10-15 minutes. Prepare spinach with little water according to package instructions. Put the spinach in a sieve and squeeze it out.
Dab meat dry, cut lengthwise and open. Season on both sides with salt and pepper. Spread 2 tablespoons of spinach on the inside of each fillet, roll it up and fix it with wooden sticks.
Heat the oil in a large frying pan. Fry the meat for 5-6 minutes, turning it over and taking it out. Remove skewers.
For the crust, grate parmesan finely. Knead the herb butter, breadcrumbs and parmesan with your hands to small flakes. Drain the potatoes, let them evaporate briefly and cut into slices. Place the potato slices and the remaining spinach in an ovenproof casserole dish.
Pour sauce over it and put rolls on top. Sprinkle herb butter crumbs over it and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 15 minutes. Remove from the oven, cut open the rolls if necessary and serve hot.