Cut the savoy cabbage into eighths, clean, cut into large pieces and wash. Cook in boiling salted water for about 5 minutes. Drain. Wash, clean and seed the tomatoes and cut them into pieces.
Cut turkey breast slices in half crosswise once each. Layer savoy cabbage, tomatoes and cold cuts in 4 small or 1 large casserole dish. For the béchamel sauce, peel and finely dice the onion. Melt the fat in a saucepan and sauté the onion.
Dust with flour and sauté briefly. Gradually add stock and milk while stirring. Let simmer for 5 minutes. Add processed cheese and let it melt in the hot sauce. Season to taste with salt and pepper.
Wash parsley and chives and dab dry. Put some chives aside for garnishing, cut the rest into fine rolls. Stir into the béchamel sauce. Spread the sauce over the vegetables. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-25 minutes.
Garnish gratin with chives put aside.