Peel, wash and slice the potatoes and carrots. Cook the potatoes and carrots in boiling salted water for 15 minutes. Add peas to the potatoes 2 minutes before the end of the cooking time.
Pour the vegetables onto a sieve, collect the vegetable water. Melt the fat in a pot. Sweat flour in it. Deglaze with 3/8 litres of vegetable water and milk while stirring. Let the sauce simmer for 5 minutes. Season to taste with salt and pepper.
Put the vegetables in a greased casserole dish and pour the sauce over them. Grate cheese and sprinkle over the casserole. Spread bacon on top. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 2) for approx. 10 minutes.