Mix the prepared sloppy joe mix with the veggie burger and Prego sauce, and set to simmer, while you boil the pasta according to the box directions.
Spread a thin layer of sauce in the bottom of two 7x11 casserole dishes. Layer red onions on top of sauce, and spinach leaves on top of that, splitting evenly between the two dishes. Using a slotted spoon, spoon the pasta out of the boiling water (so a bit of the water comes with it), in a layer on top of the spinach layers, and top off with the rest of the sauce, spooning it on and spreading it out evenly.
Place in a 400º oven for 20 minutes, or until most of the liquid is absorbed. Take out of oven, and let sit for 10 minutes before serving. perfect for potlucks, or if you like to bake ahead for emergency meals, it freezes well.