Cheddar and English mustard are a classic combination, but you can use any cheese or a mixture to make the sauce. Gruyère with Dijon mustard would be milder, while Parmesan or blue cheese (such as Stilton) would add a much stronger flavour, so you wouldn’t need quite as much.
For a meaty version, mix some fried bacon pieces or chopped up ham into the cheese sauce before pouring into the baking dish. Or go for extra veggies with half cauliflower and half broccoli florets, cooked as above.
For a crispy topping, try sprinkling 2-3 tbsp breadcrumbs over the top after grating over the cheese.