Clean, wash and cut the leek into rings. Peel and chop the garlic. Cut bread first into slices, then into cubes. Remove the rind from the cheese, grate roughly
Melt the butter in a large ovenproof pan. Roast the bread cubes in it while turning, take them out. Heat the oil in the pan. Steam leek and garlic for about 5 minutes. Season with salt and pepper. Take out about half of the leek.
Mix the leek in the pan with about half the bread and roast beef. Sprinkle with a good half of the cheese. Spread the remaining leek, bread and roast beef on top and sprinkle with the remaining cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 10 minutes. Wash the chives and dab dry. Sprinkle on the casserole and serve