Bring a large pot of water to a rolling boil and salt it generously (about 1 tbsp kosher salt). Add the penne and cook until very al dente — about 1–2 minutes less than package directions. Salting the water seasons the pasta itself; undercooking by a minute or two lets it finish cooking in the sauce and absorb flavor. Reserve 1 cup (240 ml) of the starchy pasta water before draining.

While the pasta cooks, pat the chicken dry and season both sides with 3/4 tsp kosher salt and 1/2 tsp freshly ground black pepper. Drying the chicken ensures a better Maillard sear (browned, flavorful crust).

Heat a large heavy skillet (10–12-inch) over medium-high heat. Add 1 tablespoon olive oil and 1 tablespoon butter. When the fat is shimmering and the butter foam subsides, add the chicken. Sear without moving for 3–4 minutes per side until deeply golden (internal temp ~155°F/68°C). High heat creates the crust that adds flavor through the Maillard reaction; stop short of full doneness because the chicken will finish in the sauce and retain juiciness. Transfer chicken to a plate and tent loosely with foil to rest.

Lower heat to medium and add the remaining 1 tablespoon butter to the same pan. Add the sliced onion and a pinch of salt. Cook, stirring occasionally, 6–8 minutes until softened and starting to caramelize. Slow cooking develops natural sweetness that balances the tomatoes' acidity.

Add the minced garlic and red pepper flakes (if using) and cook 30–45 seconds until fragrant — do this quickly because garlic burns and turns bitter. Then add the diced tomatoes and increase heat to medium-high. Let the tomatoes simmer briskly 6–8 minutes until they break down and reduce; use the back of a spoon to press slabs of tomato and release juices. Reducing concentrates flavor and thickens the sauce so it clings to the pasta.

Pour in 1/2 cup chicken broth to deglaze the pan, scraping up browned bits from the bottom — those fond bits are concentrated flavor. Simmer 2–3 minutes so the broth melds with the tomato base. Taste and adjust seasoning with salt and pepper.

Slice the rested chicken into 1/2-inch slices against the grain (this shortens the muscle fibers and delivers tender bites). Add sliced chicken back to the pan to warm through for 1–2 minutes; this integrates the chicken flavor into the sauce without overcooking.

Add the drained, al dente pasta directly to the skillet with the sauce. Pour in 1/2 cup of the reserved pasta water (keep more on hand). Toss vigorously over medium heat for 1–2 minutes to finish cooking the pasta and to allow the starches to emulsify with the butter and Parmesan. This is the critical step that creates a glossy, clingy sauce rather than a thin, watery one.

Remove the pan from the heat and stir in the grated Parmesan, torn basil, and lemon juice (if using). Add more pasta water a tablespoon at a time if the sauce seems dry — you want a silky texture that coats each piece of penne. Adding cheese off the heat prevents it from seizing into grainy clumps.

Taste and adjust with salt, pepper and a final drizzle of olive oil if desired. Portion into warm bowls and finish with an extra grating of Parmesan and a few basil leaves. Serve immediately so the sauce is glossy and the chicken remains juicy.