Bring a large pot of salted water to a boil. Cook pasta according to package instructions until al dente (usually 8–11 minutes). Reserve 1/2 cup pasta water, drain pasta, and set aside.

While pasta cooks, season the sliced chicken with salt, pepper, and 1/2 teaspoon Italian seasoning.

Heat 1 tablespoon olive oil in a large non-stick skillet over medium-high heat. Add the chicken slices and cook 4–6 minutes, turning once, until golden and cooked through. Remove chicken to a plate and set aside.

Reduce heat to medium. Add 2 tablespoons butter to the same pan. Sauté the shallot (or onion) for 1–2 minutes until translucent. Add minced garlic and cook 30–60 seconds until fragrant—don’t let it brown.

Pour in 1 cup chicken broth and bring to a simmer, scraping up any browned bits from the pan. Simmer for 3–4 minutes to reduce slightly.

Stir in the heavy cream and bring to a gentle simmer. Add the grated Parmesan and whisk until the cheese melts and the sauce becomes silky, about 3–5 minutes. If the sauce is too thin, simmer gently for another 2–3 minutes until it thickens.

Return the cooked chicken to the pan along with sun-dried tomatoes and baby spinach (if using). Stir to combine and heat through for 1–2 minutes. Taste and adjust salt and pepper. Add lemon zest if using to brighten the flavors.

Toss the cooked pasta into the sauce, adding reserved pasta water a splash at a time to loosen the sauce as needed. Cook together for 1–2 minutes so the pasta absorbs the sauce.

Remove from heat, sprinkle with chopped basil or parsley and extra Parmesan. Serve immediately with crushed red pepper flakes on the side.