Cook the Pasta: Fill a large pot with water and add a generous pinch of salt. Bring the water to a rolling boil, then add the linguini. Cook according to the package directions until the pasta is al dente—firm but cooked through. Before draining, reserve about half a cup of the pasta cooking water to help adjust the sauce later.
Sauté the Onion: While the pasta cooks, warm the olive oil in a large sauté pan over medium heat. Add the finely chopped onion and cook, stirring occasionally, until it softens and becomes translucent, about 3-4 minutes. This step builds a flavorful base for the sauce.
Cook the Cherry Tomatoes: Add the halved cherry tomatoes to the pan and cover with a lid. Let them cook gently for 3-4 minutes, stirring occasionally, until they start breaking down and releasing their juices.
Add Garlic and Balsamic: Stir in the minced garlic and balsamic vinegar, then continue cooking uncovered. Let the tomatoes simmer for about 8-10 minutes, stirring occasionally, until they break down fully into a rich, saucy consistency.
Stir in the Pesto: Remove the pan from heat and fold in 4 to 6 tablespoons of fresh basil pesto, mixing thoroughly to combine. The pesto adds bright herbal notes and a creamy texture to the sauce.
Combine Pasta and Sauce: Drain the cooked linguini and immediately add it to the sauté pan with the sauce. Toss everything together gently, adding reserved pasta water a little at a time if the sauce feels too thick, to loosen it up and help it cling to the pasta.
Season and Serve: Taste the pasta and adjust with sea salt and freshly ground black pepper as needed. Serve warm, garnished with extra pesto or fresh basil if desired.