Boil the pasta: Bring a large pot of salted water to a rolling boil. Add pasta and cook until just al dente. Reserve 1/3 cup of the starchy cooking water before draining.

Season the tomatoes: While the pasta cooks, place halved tomatoes in a large bowl. Add a pinch of salt and black pepper. Let them sit to draw out their juices for extra flavor.

Make the pesto sauce base: In a small bowl, combine pesto with 1–2 tablespoons of olive oil, a squeeze of lemon juice, and a spoonful of the reserved pasta water. Stir until glossy and loose. You want a pourable sauce.

Toss while warm: Add hot, drained pasta to the tomatoes. Pour over the pesto mixture and toss well. Add more pasta water, a splash at a time, until the sauce coats everything evenly.

Add the good stuff: Fold in mozzarella, Parmesan, and red onion or shallot if using. Add lemon zest and a pinch of red pepper flakes for a gentle kick.

Taste and adjust: Season with salt and pepper. If it tastes flat, add a touch more lemon juice or Parmesan. If it’s too thick, loosen with a bit more pasta water or olive oil.

Finish with basil: Right before serving, gently fold in torn basil leaves. This keeps the herbs bright and fragrant.

Rest (optional but recommended): Let the salad sit for 10–15 minutes so the flavors settle. Serve at room temperature for the best taste.