Pat the chicken dry and season both sides with salt, pepper, and Italian seasoning. (Dry chicken sears; wet chicken steams — this is the difference between golden and gray.)
Heat the olive oil and 1 tbsp butter in a large skillet over medium-high heat. Sear the chicken 5–6 minutes per side until deeply golden and cooked through (165°F at the thickest part). Remove to a plate.
Lower the heat to medium. Add the remaining 2 tbsp butter, garlic, and sun-dried tomatoes; cook 1 minute until fragrant.
Pour in the heavy cream, scraping up the browned bits from the pan — that’s pure flavor. Simmer 3–4 minutes until slightly thickened.
Stir in the parmesan until melted and smooth, then fold in the spinach and let it wilt, about 2 minutes.
Return the chicken to the skillet, spoon sauce over the top, and simmer 2 more minutes. Shower with fresh basil and serve.