Blanch broccoli and cauliflower in boiling salted water for about 5 minutes and then drain. Sweat the flour in butter and stir in the tomato paste. Pour in milk slowly and whisk in vigorously. Let the whole thing simmer gently for about 10 minutes.
Preheat the oven to 180 ° C. Put the vegetables in a greased dish and press the sausage mixture out of the skin.
Melt the cheese in the sauce and season with salt and pepper. Pour over the vegetable bratwurst mixture and bake for about 45 minutes.