Wash and clean the cauliflower and broccoli and cut into florets. Pre-cook cauliflower for about 5 minutes and broccoli for about 3 minutes in boiling water. Pour the vegetables into an ovenproof greased casserole dish.
Whisk milk and eggs, season with salt, pepper and a little nutmeg and pour on the florets. Put hot water into a fat pan. Put the casserole dish into it and let it rest in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 45 minutes.
Meanwhile peel and finely dice the onion and garlic clove. Heat the fat in a saucepan. Sauté onion and garlic in it. Wash, clean, dice and add the tomatoes. Steam for about 5 minutes at low heat.
Season to taste with lemon pepper and serve with the casserole.