Preheat the oven to 450°F (225°C). Coarsely chop the leek and onion. Chop the broccoli and cauliflower florets into ~1” chunks. Slice the sausage links into 1” pieces.
Fry onion and vegetables in butter in one pan and the sausage in a separate pan.
Put the vegetables in a baking dish, blend the mustard with the sour cream and pour over the vegetables.
Add the sausage and cheese on top and season with thyme.
Bake in oven on upper rack for 15 minutes.