once the sauce has thickened, stir in gruyere and cheddar, then add mustard, nutmeg and salt & pepper to taste. stir in roasted broccoli/cauliflower. pour back into the baking dish, sprinkle over the rest of your cheddar (1/2cup/50g) and pop back in the oven at the same temp for 20mins or until golden and bubbly.