In a large skillet, mix together spaghetti sauce, peppers, onions (if you are using them) and drained tomatoes.
Set your sausage links in the sauce.
Cover and simmer on med-low for 45 minutes.
If sauce is thick enough for you, move on to the next step. Otherwise, if you prefer a thicker sauce, remove the sausages and thicken your marinara sauce by adding 1 tsp of cornstarch mixed with 1 tbsp cold water to your sauce, turn up to med-high heat and bring to a boil, return it to a simmer for 5 more minutes and it should be plenty thick.
To serve, place one sausage on your plate, cover with 1/2 cup of sauce and sprinkle with the cheeses to your liking (cheese will melt immediately). Note that you can serve these in buns/rolls if you like!