Step 1: Making the béchamel sauce.
In a saucepan, melt the butter and add the flour. Once the flour has absorbed the butter, remove from the heat and add the chicken stock, whisk well, and put back on the heat until it boils.
Add the crème Fraiche and cook for a few minutes off the heat. Add the mascarpone, adjust the seasoning and add a pinch of nutmeg (set aside).
Step 2: Cooking the meat.
In a stewpot, pour a dash of olive oil and brown the onion and carrot, then add the meat, adjust the seasoning and cook for 5 minutes, then put it in a colander to remove the fat.

Step 3: Preparation of the sauce.
In the same pot, heat a little oil and brown the leek, then add the peppers, the crushed tomatoes, the tomato paste, and the white wine, cook for 2 minutes and add the meat, the bay leaf, the oregano, salt, and pepper (add a little chicken stock if necessary) and simmer for 10 minutes over medium heat.

Step 4: Assembly.
In a gratin dish put 1 layer of lasagna pasta then 1 layer of béchamel and 1 layer of meat, repeat the operation and finish with a layer of béchamel, sprinkle with grated Gruyere. Cook in a preheated oven at 180o for 35 to 40 min.