Cook the pasta in a large pot of boiling salted water, per package directions. Drain, rinse, and put back into the pot.
Add the potatoes, carrots, and onions to a pan of boiling water. Cook 6-8 minutes until soft. Drain, reserving 2 cups of the cooking water.
Add the cooked vegetables, the cooking water, the cashews, yeast, and seasonings to a blender, and blend until creamy and smooth. This is the cheez sauce. Pour over the pasta.
Sauté the bell pepper and green onion for a few minutes. Mix into the pasta & sauce.
Transfer the mac & cheez to a serving bowl and top with vegan parm.