Fry the meat in the hot oil until crumbly. Fry the diced onion and diced garlic briefly. Add the stock and tomato paste. Let simmer for about 4 minutes.
Wash the chives and cut into rolls. Stir half into the puree from the day before.
Fold the drained mushrooms under the minced meat. Season with salt and pepper. Pour the mince into a refractory tin. Spread mashed potatoes on top.
Sprinkle cheese on the puree. Bake in a preheated oven (fan oven: 200 ° C) for about 8 minutes. Sprinkle with the remaining chives.