Salt the water: Bring a large pot of salted water to a boil. The water should taste like the sea; this is your main chance to season the pasta.
Prep the chicken: Pat dry, season with salt and pepper, and set aside.
If using thighs, trim excess fat for even cooking.
Cook the pasta: Add pasta to boiling water and cook until just shy of al dente. Reserve 1 cup of pasta water before draining.
Sear the chicken: Heat 1 tablespoon olive oil in a large skillet over medium-high. Add chicken in a single layer and sear until golden and cooked through, 5–7 minutes.
Don’t overcrowd; cook in batches if needed. Transfer to a plate.
Sauté aromatics: Lower heat to medium. Add remaining oil and the garlic.
Cook 30–60 seconds until fragrant, not browned.
Create the sauce base: Stir in cream or 1/2 cup pasta water to the skillet. Simmer briefly to warm through and emulsify.
Add pesto: Off the heat, whisk in the pesto. Return to low heat and adjust consistency with more pasta water as needed.
The sauce should lightly coat a spoon.
Toss it all together: Add drained pasta and cooked chicken to the skillet. Toss gently to coat. Add Parmesan, lemon zest, and a squeeze of lemon juice.
If using tomatoes, fold them in now. Season with salt, black pepper, and red pepper flakes.
Finish and serve: Top with toasted pine nuts, extra Parmesan, and fresh basil. Serve immediately while glossy and hot.