Grate the aged cheddar, gruyère and fontina and mix them together in a bowl. Return the bowl to the refrigerator while you prepare the liquid base so the cheese stays cold (cold cheese melts more smoothly).

Measure and combine the flour and cornstarch in a small bowl and set aside. Measure milk, cream, wine and seasonings so everything is ready.

Make the Base

Warm aromatics and thicken

Place a medium saucepan over medium-low heat and add the butter. When the butter is melted and foaming, add the flour and cornstarch. Stir constantly with a whisk or wooden spoon for 1 minute to remove the raw flour taste (do not let it brown).

Add liquids

Slowly whisk in the milk and heavy cream in a thin stream so lumps don't form. If using white wine, stir it in now. Continue whisking until the mixture is smooth and just beginning to steam (do not boil).

Bring the base to a gentle simmer for 2–3 minutes, stirring occasionally, until it thickens enough to coat the back of a spoon.

Incorporate Cheese

Melt the cheese slowly

Reduce heat to low. Add about one-quarter of the shredded cheese to the hot cream base and whisk until fully melted and smooth before adding the next portion. Continue adding the cheese in batches, whisking smooth between additions to keep the sauce velvety and avoid separation.

Season and finish

When all the cheese is incorporated and the sauce is smooth, whisk in the Dijon mustard, Worcestershire sauce, black pepper, nutmeg (if using) and salt to taste. If the sauce seems too thick, stir in up to 2 tablespoons of additional milk, one tablespoon at a time, until desired consistency is reached. If the cheese tastes flat, add the optional 1/2 teaspoon lemon juice and taste again.

Serve

Transfer and garnish

Transfer the melted cheese to a warmed serving bowl or a small fondue pot set on low heat. Sprinkle with chopped chives or parsley.

Serve immediately with crusty bread, toasted vegetables or chips. Stir occasionally while serving to keep smooth; if sauce thickens, stir in a splash of warm milk or cream to loosen.