Boil the Pasta: Fill a large pot with water, salt it generously, and bring it to a rolling boil. Add the linguini and cook according to the package directions until al dente. Just before draining, scoop out about ½ cup of the starchy pasta water and set it aside. Drain the pasta and set it aside.
Sauté the Onion: In a large sauté pan set over medium heat, warm the olive oil. Once heated, add the chopped onion and cook for about 2–3 minutes, stirring occasionally, until softened and translucent.
Simmer the Tomatoes: Add the halved cherry tomatoes to the pan. Cover with a lid and let them cook undisturbed for about 2 minutes. Once they begin to soften, remove the lid, stir in the minced garlic and balsamic vinegar, and continue to cook for another 8–10 minutes. Stir occasionally, allowing the tomatoes to break down and release their juices, creating a rustic, sauce-like consistency.
Add the Pesto: Turn the heat to low and stir in 4 to 6 tablespoons of pesto, depending on your taste preference. Mix thoroughly until the pesto is evenly distributed and the sauce is rich and fragrant.
Combine Pasta with Sauce: Add the drained linguini to the pan with the tomato-pesto mixture. Toss gently but thoroughly to coat the pasta evenly. If the sauce feels too thick, gradually add a bit of the reserved pasta water until the desired consistency is reached.
Season and Serve: Taste and adjust seasoning with sea salt and freshly ground pepper as needed. Serve immediately while hot, optionally garnished with fresh basil, grated cheese, or extra pesto.