Flatten and season the chicken: If using thick breasts, slice into cutlets or pound to an even 1/2-inch thickness. Pat dry, then season both sides with salt and pepper.
Sear for golden color: Heat 1–2 tablespoons olive oil in a large skillet over medium-high.
Add chicken in a single layer. Cook 3–5 minutes per side until golden and almost cooked through. Transfer to a plate and tent with foil.
Start the sauce base: Reduce heat to medium.
Add 1 tablespoon butter. Stir in garlic and red pepper flakes. Cook 30–45 seconds until fragrant; don’t let the garlic brown.
Deglaze and simmer: Pour in chicken broth, scraping up any browned bits.
Simmer 2–3 minutes to reduce slightly.
Make it creamy: Stir in heavy cream, sun-dried tomatoes, and Italian seasoning. Bring to a gentle simmer; cook 3–4 minutes to thicken.
Add Parmesan and adjust: Reduce heat to low. Stir in Parmesan until smooth.
Taste and season with salt and pepper. If sauce is too thick, add a splash of broth.
Finish the chicken: Return chicken and any juices to the skillet. Nestle into the sauce and simmer 3–5 minutes until cooked through (internal temp 165°F/74°C) and sauce clings.
Brighten and garnish: Stir in lemon juice to taste.
Add fresh basil. If using spinach, fold it in now to wilt.
Serve: Spoon extra sauce over the chicken. Pair with zucchini noodles, cauliflower mash, roasted asparagus, or a simple green salad.