Bring 1 inch of water to a boil in a large pot.

Place broccoli in a steamer basket and set over water.

Cover and steam until crisp-tender, about 5 to 6 minutes; drain and return to pot.

Alternatively, you can cook broccoli in microwave.

Meanwhile, in a medium saucepan, whisk together milk and flour until blended; place over medium-high heat and bring to a boil, stirring often. Reduce heat to low and simmer until slightly thickened, about 3 minutes; remove from heat. Whisk in cheese until melted; whisk in mustard and salt.

Pour sauce over broccoli and toss until coated.

Yields about 1 cup per serving.