Ask a GH (Grown-Up Helper) to bring a large pot of water to a boil, salt it, add the cauliflower and cook until crisp-tender, about 5 minutes. Ask your GH to transfer the florets with a slotted spoon to a large bowl. In the same boiling water, cook the pasta until al dente, then drain in a colander. Add the pasta to the cauliflower.
While the pasta is working, ask your GH to melt the butter in a saucepan over medium heat. Have your GH help you whisk in the flour and cook for 1 minute, then whisk in the milk and chicken broth and cook until thickened, about 5 minutes. Stir in the cheese. Whisk in the mustard and season with pepper. Stir the cheese sauce into the pasta and cauliflower. Yum-o!