Bring a large pot of generously salted water to a boil. Cook pasta according to package directions until al dente (typically 1-2 minutes less than instructed). Reserve ½ cup of pasta water before draining. Do not rinse the pasta.

While the pasta cooks, season chicken pieces with salt and pepper. In a large skillet or Dutch oven, heat 2 tablespoons olive oil over medium-high heat until shimmering. Add the chicken in a single layer, working in batches if necessary to avoid overcrowding. Cook for 3-4 minutes per side until golden brown and cooked through (165°F internal temperature). Transfer to a plate and set aside.

In the same pan, reduce heat to medium and add remaining tablespoon of olive oil and butter. Once butter has melted, add diced onion and cook for 2-3 minutes until softened and translucent.

Add minced garlic and cook for 30 seconds until fragrant, being careful not to burn it as this will create bitterness in your sauce.

Pour in chicken broth, scraping the bottom of the pan with a wooden spoon to release any flavorful browned bits (this deglazing step adds tremendous depth to your sauce).

Reduce heat to medium-low and stir in heavy cream, Italian herbs, and red pepper flakes if using. Bring to a gentle simmer (never a boil, which can cause the cream to separate).

Let the sauce reduce for 3-4 minutes, stirring occasionally, until it begins to thicken slightly. The sauce should coat the back of a spoon but still be fluid.

Add the grated Parmesan cheese in small handfuls, stirring after each addition until fully melted and incorporated into the sauce.

Return the cooked chicken to the pan, including any accumulated juices, and stir to coat with the sauce.

Add the fresh spinach in batches, stirring until wilted before adding more. The spinach will significantly reduce in volume as it cooks.

Add the drained pasta directly to the sauce and toss gently but thoroughly to coat every strand. If the sauce seems too thick, add reserved pasta water a tablespoon at a time until you reach your desired consistency.

Taste and adjust seasoning with additional salt and pepper if needed. Serve immediately, garnished with extra Parmesan cheese if desired.