Fill a large pot with lightly salted water and bring to a rolling boil. Stir in pasta and return to a boil. Cook pasta uncovered, stirring occasionally, until tender yet firm to the bite, about 8 to 10 minutes. Drain and transfer into a large bowl.

Meanwhile, heat oil in a frying pan over medium-low heat. Add onion; cook and stir until softened, about 3 minutes. Stir in pesto, salt, and pepper until warmed through.

Add pesto mixture to hot pasta; stir in grated cheese and toss well to coat.