Prepare the pasta according to the package directions, but 3-5 minutes before the pasta is done, drop in the cauliflower to cook. Drain.
Meanwhile, heat the butter in a skillet; add the leek and sauté until tender.
Add the flour and stir to blend.
Add the milk all at once and whisk together. Bring to a boil.
Remove from the heat and add the sour cream, tomato and mustard.
Combine the sauce with the pasta.
Lay half the pasta in a 9x 13 pan.
Sprinkle with half the gruyere.
Add the rest of the pasta.
Sprinkle with the remaining cheeses.
Bake at 350 until heated through.