Bring a large pot of water to boil. Add cauliflower and cook until it is easily pierced with a fork. Use a slotted spoon to transfer the cauliflower to a large mixing bowl.

In the same pot of boiling water, cook the pasta according to package directions. Drain and add to the cauliflower.

In a saucepan, melt butter and cook for1 minute.

Whisk the flour into the butter for 1minute. Whisk in milk and cook until thickened, about 5 minutes. Remove from heat and stir in the mustard and cheese.

Mix the cheese sauce with the cauliflower and pasta. Season with salt and pepper.