Wash and clean the broccoli and cauliflower and cut into florets. Blanch in boiling salted water for about 4 minutes. Drain the vegetables, collecting the cooking water. Rinse the cabbage florets in cold water and let them drip off.
Peel and chop the onion. Grate cheese finely. Melt butter in a pot. Steam onion in it until transparent. Add flour and sweat briefly. Gradually stir in milk and 1⁄4 l cooking water. Bring sauce to the boil while stirring and simmer for about 5 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Stir 50 g cheese into the sauce and melt. Season to taste with salt, pepper, nutmeg and lemon juice. Put the vegetables in an ovenproof casserole dish.
Pour cheese sauce over it and sprinkle with the rest of the cheese. Bake in a hot oven for 20-25 minutes.
Cut bread into small cubes. Wash the thyme, shake dry and pluck the leaves from the stalks. Coarsely chop the almonds. Mix bread, thyme, almonds and 1⁄2 tsp. salt. Switch the oven to grill level or highest temperature.
Sprinkle the casserole with the bread mixture and bake under the hot grill for 3-5 minutes until crispy.