Prep the chicken: pat the chicken breasts dry with paper towels, then slice each into 3/4-inch (2 cm) strips. Season both sides with 1 teaspoon salt and 1/2 teaspoon black pepper. Drying and cutting the chicken ensures it browns quickly and evenly instead of steaming—browning adds flavor via the Maillard reaction.

Bring a large pot of salted water to a vigorous boil (use about 1 tablespoon salt per 4 quarts of water). Add the pasta and cook 1–2 minutes less than package directions for al dente; drain, reserving 1 cup of the pasta cooking water. Undercooking slightly allows the pasta to finish cooking in the sauce and absorb flavor without getting mushy.

While pasta cooks, heat a large 12-inch skillet over medium-high. Add 2 tablespoons of olive oil and let it shimmer (this preheating ensures immediate searing contact). Add the chicken strips in a single layer—work in two batches if needed so pieces don’t crowd—and cook without moving for 2–3 minutes until a golden crust forms. Flip and cook another 1–2 minutes until just cooked through (internal temp 165°F / 74°C). Transfer chicken to a plate and tent with foil. Searing develops a flavorful crust and seals in juices.

Reduce heat to medium, add the remaining 2 tablespoons olive oil to the same skillet. Add the diced onion and a pinch of salt; sauté 3–4 minutes until softened and translucent. Add the minced garlic and red pepper flakes and cook for 30–45 seconds until fragrant. Sweating the onion builds a sweet base; briefly cooking garlic releases its aromatics without burning (burnt garlic tastes bitter).

Pour in 1/2 cup chicken broth (or white wine if using) and scrape up the brown bits from the pan with a wooden spoon—those bits are concentrated flavor (fond). Let the liquid simmer and reduce by about half, 2–3 minutes. Add the lemon zest and juice to brighten the sauce; acid balances the fat and carries flavor.

Return the chicken and any collected juices to the pan. Add the butter and about 1/2 cup of the reserved pasta water. Stir and let the butter melt, then add the drained pasta. Toss vigorously for 1–2 minutes to emulsify the sauce: the starch in the pasta water binds the oil and butter into a glossy coating that clings to the pasta. If the sauce seems dry, add more reserved pasta water 1 tablespoon at a time.

Turn off the heat and stir in the grated Parmesan, chopped parsley, and a few grinds of black pepper. Taste and adjust seasoning with salt, lemon juice, or red pepper flakes. Resting off-heat before adding cheese prevents it from clumping and creates a creamier sauce.

Divide among four warmed bowls, finish with an extra grind of black pepper, a light drizzle of olive oil, and an optional sprinkle of additional Parmesan and parsley. Serve immediately. Warming the bowls keeps the pasta from cooling too quickly and maintains the best texture.