Preheat your oven to 375° F (190° C). Then start boiling the rigatoni in salted water until it is al dente (soft, but still very firm) then drain and rinse with cold water.
While the pasta is cooking, chop up the veggies of your choice into very small pieces. (A food processor works great for this. Just wash the veggies and toss them in. Pulse a few times until everything is finely chopped). (You can use whatever veggies that you have on hand, but about 3-4 cups of chopped vegetables work well.)
Add a little oil to the bottom of a large skillet and sauté diced onions for about 3-5 minutes until they become translucent.
Add the chopped veggies and sauté over medium-low heat for about 5 more minutes until they are soft.
Put in the package of vegan beef crumbles (or 3 veggie burgers or vegan sausage, chopped into small pieces) and cook over medium-high heat for an additional 5 minutes.
Pour in 2 jars of marinara sauce, 1 cup starchy pasta water (from cooking noodles), 1 tsp of salt, 1 tbsp Italian seasoning, and a sprinkle of sugar (if needed), and let simmer on the lowest setting.
Add the drained, cooked pasta to the sauce and stir well until combined.
Assemble the rigatoni
Pour half of the sauce and noodles into the bottom of a 9 X 13-inch casserole dish.
Add a layer of vegan mozzarella cheese (about 1 1/2 cups), then pour the remaining noodles and sauce over that layer of cheese.
Top your rigatoni with an additional 1 1/2 cups of vegan mozzarella cheese or a batch of vegan bechamel sauce.
Bake the pasta
Bake at 375° F (190° C) covered with foil for 45 minutes, then take the foil off and bake for another 15-20 minutes until bubbly and slightly browned on top.
Let it cool for about 10 minutes to firm up a little before spooning the baked rigatoni onto a plate.
Notes
If using vegan cheese, be sure to cover the rigatoni with foil for the first 45 minutes of baking to help the cheese melt.