Clean and wash the beans, cut them in half once and steam them in little boiling salted water for about 10 minutes. Peel, wash and quarter potatoes. Heat 2 tablespoons of oil in a frying pan and fry the potatoes for about 10 minutes, turning several times. Drain the beans and chill in cold water. Peel and finely dice the onion.
Heat 1 tablespoon of oil in a frying pan and fry the minced meat until crumbly. Add onion and thyme and fry briefly. Add chilli sauce and 100 ml water, simmer until the liquid has almost evaporated. Season to taste with salt and pepper. Wash, clean and halve the tomatoes. Season potatoes with salt and pepper. Spread the potatoes, beans, minced meat and tomatoes in an ovenproof dish. Melt the fat in a saucepan and dust with flour. Sweat while stirring, gradually adding milk and 300 ml water. Bring to the boil, stir in stock and simmer for 5 minutes.
Spread the potatoes, beans, minced meat and tomatoes in an ovenproof dish. Melt the fat in a saucepan and dust with flour. Sweat while stirring, gradually adding milk and 300 ml water. Bring to the boil, stir in stock and simmer for 5 minutes. Season sauce with salt, pepper, lemon peel and juice. Pour sauce over the casserole. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-25 minutes