Prep the chicken: Pat the chicken breasts dry and cut each into 1/2" (1–1.5 cm) thick slices or bite-sized strips. Season both sides with 1 tsp salt, 1/4 tsp black pepper and 1/2 tsp Italian seasoning. Dry chicken sears better — the patting removes surface moisture so you get a golden crust (Maillard reaction) that locks in juices and adds flavor.

Bring a large pot of salted water to a rolling boil and cook the pasta to 1–2 minutes before package directions (al dente). Reserve 1 cup of the starchy pasta cooking water before draining. Cooking pasta slightly under allows it to finish cooking in the sauce so the texture is perfect; the reserved water is a natural emulsifier to loosen or thicken the sauce as needed.

Heat a large skillet over medium-high heat. Add the olive oil and 1/2 tbsp butter. When the fat shimmers, add chicken in a single layer (don't crowd — cook in two batches if needed). Sear without moving for 2–3 minutes per side until deeply golden and cooked through (internal temp 165°F / 74°C). Searing creates flavor; quick cooking keeps the chicken moist. Transfer chicken to a plate and tent with foil to rest (resting redistributes juices).

Lower heat to medium and add the remaining 1/2 tbsp butter to the same skillet. Add the diced onion and a pinch of salt; sauté until translucent and just starting to color, about 4–5 minutes. The salt draws out moisture and helps the onion soften evenly. Add the minced garlic and red pepper flakes and cook 30–45 seconds until fragrant (blooming the garlic briefly releases aromatic oils; don’t let it brown or it will taste bitter).

Deglaze the pan with the chicken stock, scraping up the browned bits (fond) with a wooden spoon. Let the stock reduce by about one-third — 2–3 minutes. Deglazing captures the concentrated flavor left from searing the chicken and forms the base for the sauce.

Stir in the heavy cream and Italian seasoning and bring the mixture to a gentle simmer (not a rolling boil). Simmer 2–3 minutes to slightly thicken. Simmering reduces the raw cream taste and concentrates flavor; a gentle simmer prevents the cream from separating.

Turn the heat to low. Whisk in the Greek yogurt until smooth, then add the grated Parmesan a handful at a time, whisking to create a glossy sauce. Tempering the yogurt by lowering the heat avoids curdling; add cheese gradually so it melts evenly and the sauce emulsifies (cheese + starch from pasta water gives a satiny texture).

Add the cooked pasta, seared chicken, lemon juice, and about 1/2 cup reserved pasta water to the skillet. Toss gently over low heat until the pasta is evenly coated and sauce clings to each piece, adding more pasta water by the tablespoon if needed to reach desired consistency. The starch in pasta water binds fat and cheese into a silky sauce that adheres to the pasta.

Taste and adjust seasoning with remaining salt and pepper. Finish with chopped parsley and an extra sprinkle of Parmesan if desired. Serve immediately. Letting the assembled pasta sit too long will thicken the sauce; serve right away for the best texture.