Prep the chicken: butterfly each chicken breast (slice horizontally but don’t cut through), then lay flat and place between plastic wrap and pound to an even 1/2-inch thickness. Trim any large pieces of fat. Season both sides with 1 tsp kosher salt and 1/2 tsp black pepper. Why: pounding to an even thickness ensures the chicken cooks evenly and quickly so the exterior crisps before the interior dries out.

Dust chicken lightly in the 1/4 cup flour, shaking off excess. The thin flour coating helps create a golden crust and gives the sauce a little extra body from the pan fond when deglazed.

Bring a large pot of salted water to a rolling boil (use about 1 tbsp kosher salt per 4 quarts). Add the pasta and cook until 1–2 minutes shy of package directions for al dente. Reserve 1 cup pasta cooking water before draining. Why: finishing pasta in the sauce lets the noodles absorb flavor and the starchy cooking water helps bind the sauce and thicken it naturally.

Heat a large skillet over medium-high heat, add 1 tbsp olive oil and 1 tbsp butter until shimmering. Sear the floured chicken in batches (do not overcrowd) 3–4 minutes per side, or until golden brown and just cooked through (internal temp ~160°F/71°C — it will rise to 165°F while resting). Transfer to a plate and tent with foil. Why: high initial heat creates the Maillard crust for flavor; finishing in the sauce prevents overcooking while keeping juices intact.

Reduce heat to medium and add the remaining 1 tbsp olive oil and 1 tbsp butter to the skillet. Add the diced onion and sauté 4–5 minutes until translucent and slightly golden — this mellows the onion’s sharpness and builds a savory base. Stir in the minced garlic and red pepper flakes and cook 30 seconds until fragrant. Why: lightly cooking garlic releases aromatic oils; avoid browning it too much to prevent bitterness.

Deglaze the pan with the white wine (or 1/4 cup chicken stock) scraping up any browned bits from the bottom — those browned bits (fond) are concentrated flavor. Let the wine reduce by half, about 1–2 minutes.

Add the remaining 3/4 cup chicken stock and bring to a gentle simmer. Stir in the heavy cream and lemon juice, then simmer 2–3 minutes to slightly thicken. Why: reducing a little concentrates flavor and the cream plus lemon give a balanced, silky sauce — the acid from lemon brightens and cuts through the richness.

Stir in 1/2 cup of the grated Parmesan until melted and smooth. Taste and adjust seasoning with remaining 1/2 tsp kosher salt and 1/2 tsp black pepper (or to taste). If the sauce seems too thick, loosen with reserved pasta water a few tablespoons at a time. Why: adding cheese off-heat and stirring ensures it melts into the sauce without clumping; starch from the pasta water helps emulsify the sauce for a glossy finish.

Slice the rested chicken into 1/2-inch strips. Add the drained pasta directly to the skillet and toss to coat, adding reserved pasta water in small splashes until the sauce clings to the noodles (about 1/4–1/2 cup). Add the sliced chicken and toss gently to combine and heat through for 1 minute. Why: finishing everything together lets flavors marry and ensures the chicken is hot without overcooking.

Remove from heat and stir in the remaining 1/4 cup Parmesan and chopped parsley for brightness. Plate immediately, finishing with an extra grind of black pepper, a light drizzle of olive oil or an extra sprinkle of Parmesan if desired. Why: the final cheese and herbs add freshness and a silky texture; serve hot so the sauce remains glossy.