To prepare the Bechamel sauce, melt the butter in a heavy saucepan on medium heat. Sprinkle in the flour, whisking or stirring constantly, until a smooth pasteis formed. Add the heated milk slowly, a cup at a time, continuing to whisk until the sauce begins to thicken. Add the mustard, nutmeg, salt, and pepper and let the sauce thicken further on low heat. Remove from heat and cover.
Saute the onions, garlic, and basil in oil. When the onions are translucent, add the cauliflower and saute for 5 minutes or more. Add the tomatoes and simmer until the cauliflower is tender.
Cook the pasta al dente and then drain.
In a large bowl, combine the cooked pasta, the Sauteed vegetables, and the lemon juice. Mix in the Bechamel sauce. Put half of the pasta mixture in an oiled baking dish. Sprinkle on half of each of the cheeses. Add the rest of the pasta mixture and then the rest of the cheese. Top with seasoned bread crumbs and bake covered at 375 degrees for about 30 minutes and then uncovered for about 15 minutes or until bubbling and golden on top