Bring a large pot of water to a boil and salt it with 1 tsp kosher salt.

Cook 12 oz fettuccine until al dente (usually 9–11 minutes). Reserve 1/2 cup pasta water, then drain.

Slice chicken breasts in half lengthwise or into 1/2-inch cutlets. Pat dry and season with 1/4 tsp kosher salt and 1/2 tsp black pepper.

Heat 1 tbsp olive oil and 1 tbsp butter in a large skillet over medium-high heat.

Add chicken and sear for 3–4 minutes per side until golden and just cooked through (165°F internal). Transfer to a cutting board and let rest for 3 minutes, then slice.

Reduce heat to medium and add remaining 2 tbsp butter to the skillet. When melted, add minced garlic and cook for 20–30 seconds until fragrant.

Pour in 1.5 cups heavy cream and 1/2 cup low-sodium chicken broth. Stir and bring to a gentle simmer, reducing heat to low and simmering for 3–4 minutes to slightly thicken.

Remove skillet from heat and gradually whisk in Parmesan until the sauce is smooth. Add lemon juice and taste for seasoning. If sauce is too thick, add reserved pasta water gradually until glossy.

Add drained pasta and sliced chicken to the skillet. Toss to coat evenly and warm everything through for 1–2 minutes.

Sprinkle with chopped parsley and red pepper flakes if desired. Serve immediately with extra Parmesan.