Heat a couple tablespoons of olive oil and add one minced onion. It doesn't HAVE to be minced, just finely chopped. Let brown and then add the pasta sauce. Season with pepper and oregano (if you'd like, I usually don't because I think it overpowers). Dilute the sauce by rinsing out the jar with water (about a half cup) and then adding that to the pot. Waste not, want not. Add a few tablespoons chopped parsley, but reserve some to sprinkle over the final product. It doesn't just add color, it also adds flavor in two dimensions. Cooked parsley has a different taste than fresh.
Cook down for about 10-15 minutes and you're good to go. You can dilute it more and keep cooking it down if you want, too.