Clean, halve, wash and slice the fennel. Put some fennel green aside. Peel, wash and slice the carrots. Cover the carrots and fennel and cook them for about 5 minutes in little boiling salted water.
Drain the vegetables and let them drain. Peel and chop onion. Heat oil in a pan. Fry the minced meat and onion for about 5 minutes until crumbly. Season mince with salt and pepper. Put vegetables and minced meat in a greased casserole dish (26 cm Ø).
For the béchamel, melt the fat in a pot. Dust with flour and sauté briefly while stirring. Stir in the milk and stock little by little. Simmer for 5 minutes. Gradually stir the cheese into the béchamel and melt it.
Season the béchamel with salt, pepper and nutmeg and spread over the vegetables. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 25-30 minutes. Garnish with fennel greens.