Boil the pasta: Bring a large pot of well-salted water to a rolling boil. Cook the macaroni until just shy of al dente (usually 1 minute less than the package). Drain and set aside. Toss with a touch of butter if you’re worried about sticking.

Warm the milk: In a small pot or microwave-safe cup, gently warm the milk until steamy, not boiling. Warm milk blends more smoothly into the roux.

Make the roux: In a large saucepan over medium heat, melt the butter. Whisk in the flour and cook for 1–2 minutes, whisking constantly, until the mixture smells nutty but is still pale. Don’t let it brown.

Add the milk gradually: Slowly pour in the warm milk while whisking. Keep whisking until the sauce is smooth. Bring to a gentle simmer and cook 3–5 minutes until slightly thick and glossy. It should coat the back of a spoon.

Season the base: Stir in the mustard, a good pinch of salt, black pepper, and a pinch of paprika or cayenne if using. Taste the sauce; it should be pleasantly seasoned before cheese goes in.

Add the cheese off heat: Turn off the heat. Add the cheddar and Gruyère/Jack in small handfuls, whisking after each addition until melted and smooth. If the sauce seems too thick, whisk in a splash more milk.

Combine with pasta: Fold the cooked pasta into the cheese sauce until every piece is coated. If it tightens up, loosen with a bit of warm milk.

Adjust and serve: Taste and adjust salt and pepper. Serve hot, with a sprinkle of extra pepper or paprika if you like.