Thinly slice onion; heat 4 tbsp butter in skillet until melted then add onion + 1 cup sauce; simmer 18-20 min until onion is softened; season with salt/pep

Meanwhile, boil a large pot of salted water, add half the box of pasta + cook 10-12 min or until al dente, then drain + return to pot

Toss pasta with buttery tomato sauce + divide between plates