Bring a large pot of water to a boil and add the tablespoon salt. Add macaroni and cook according to its package, adding broccoli for the final 3 minutes.
While the pasta cooks, melt butter in a saucepan and whisk in flour for 1 minute. Gradually whisk in milk, then add sauce salt, pepper and mustard.
Simmer for 4 minutes until lightly thickened, then remove from the heat and stir in cheddar until smooth.
Drain pasta and broccoli thoroughly, shaking the colander to release water trapped in the florets.
Fold into the sauce, let stand for 1 minute and serve while the broccoli is tender and the sauce still flows.