Heat large pot of lightly salted water to boiling. Add pasta and cook 6 minutes. Add broccoli and cook an additional 4 minutes. Drain.Advertisement
Meanwhile, whisk milk and flour together in a small saucepan. Bring to a simmer over medium heat, then simmer 3 minutes, until thickened. Remove from heat and whisk in mustard, salt, pepper and cayenne.
Add grated Cheddar, whisking constantly until melted. Transfer pasta and broccoli to serving bowl. Pour cheese sauce over pasta-broccoli mixture and stir until coated.