Heat a large pot of water to a boil and add the pasta shells. Cook according to package directions.
While the pasta is cooking, add the broccoli to a microwave safe bowl along with 2 tablespoons of water. Cover tightly with plastic wrap and microwave for 3 minutes. Let sit for 2 minutes and then carefully remove the plastic wrap and drain the liquid from the broccoli.
Heat the butter over medium heat in a large saucepan until melted. Whisk in the flour and cook for 1 minute.
Whisk the milk and cream into the flour mixture until smooth and creamy. Continue cooking and whisking until the mixture has thickened a bit.
Remove from the heat and stir in the cheese, mustard powder, salt, and pepper. Continue stirring until the cheese has melted and the sauce is smooth and creamy.
Add the drained pasta and broccoli to the cheese sauce and stir to coat.
Serve hot.
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Notes
We like shells for the maximum amount of cheese sauce per bite, but macaroni works just fine.We much prefer the flavor and texture of fresh broccoli to frozen broccoli. Use fresh if available. Chop the broccoli into smaller pieces so that it stirs into the pasta nicely. You can swap the white cheddar for Gruyere, Monterey jack, or smoked gouda, if preferred.