Cook your pasta of choice according to the package directions. (Reserve 1 3/4 cups of the cooking water.)
While your pasta is cooking, peel and dice an onion, seed and dice the red bell pepper, and mince three cloves of garlic. Then saute them over low heat in a little water with a sprinkle of salt until the onions turn translucent.
Cut open the package of silken tofu and drain off any excess water, then add the tofu to the blender. (There is no need to press silken tofu.) Then add the sauteed onion, pepper, garlic, nutritional yeast, lemon juice, mustard, salt, and red pepper flakes.
Add 1 cup of the starchy pasta water to the blender and blend until smooth. Add an additional 3/4 cup of the pasta water and blend again until very smooth.
Put the cooked, drained pasta into the frying pan used to saute the onions and pour the tofu mac & cheese sauce over the macaroni.
Mix well and warm over low heat.
Serve hot. The sauce will thicken slightly as it cools.