Bring a large pot of well salted water to a boil and cook the pasta according to package directions.
Heat a large saute pan over medium heat and add the olive oil. When the oil is heated add the diced onion and saute for a couple of minutes until its softens.
Add the cherry tomatoes and cover. Let the tomatoes cook for a couple of minutes until they soften then add the balsamic vinegar and garlic.
Saute while stirring occasionally for about 10 minutes until the tomatoes have fully softened and become saucey.
Add the pesto to the sauce and stir well to mix it in.
Remove a ½ cup of cooking water from the pasta then drain.
Add the pasta to the sauce in the saute pan and mix well adding in some of the reserved pasta water to loosed up the sauce if it's too tight.