Bring a pot of salted water to a boil, and prepare the noodles according to the directions on the package.
While you wait for the water to come to a boil, slice the red onion and cherry tomatoes.
Once the pasta is cooking, prepare the pesto, if you haven't already. Add the pesto ingredients to a blender, and blend until smooth.
When the pasta is tender, reserve a 1/4 cup of the starchy cooking water, then drain the pasta.
In a large skillet over medium-high heat, add a tablespoon of olive oil and start sautéing the red onion. It should soften in about 5 minutes.
Then add in the cherry tomatoes and stir briefly. Season with a 1/4 teaspoon of salt as you stir the veggies.
When the veggies are as tender as you'd like them to be, add in the pasta, prepared pesto, and the cooking water to help thin out the pesto sauce, if needed.
Stir until everything is hot and evenly coated, then season with salt and pepper to taste.
