Fill a large stock pot with salted water and turn it on high. Once it is boiling, boil your pasta for the recommended time on the package. Reserve ½ cup of the pasta water and drain the rest.
While the pasta boils, heat the olive oil over medium heat and add the onion. Soften for 1-2 minutes. Then, add the Italian sausage and cook until browned. You can then either drain the fat, or keep it. I prefer to keep it for flavor.
Add the garlic, marinara sauce, coconut milk, nutritional yeast and pasta water. Once that has been stirred, add in the spinach and cook until it's wilted. (I prefer the spinach leaves whole, but for little kids, I find it's easier if it's torn.)
Finally, add in the al-dente pasta and stir to coat.
Serve warm. Leftovers keep well 1-2 days in an air-tight container.