Season the chicken: Pat the chicken dry.
Season both sides with salt, pepper, and half the Italian seasoning.
Sear for extra flavor (optional but recommended): Heat olive oil and butter in a skillet over medium-high. Sear chicken 2–3 minutes per side until lightly golden. Transfer to the slow cooker.
Build the sauce base: Add minced garlic to the skillet for 30 seconds.
Pour in chicken broth to deglaze, scraping up browned bits. Transfer this liquid to the slow cooker.
Add flavor boosters: Stir in remaining Italian seasoning, red pepper flakes, and sun-dried tomatoes. Pour in the heavy cream.
Slow cook: Cover and cook on Low for 3.5–5 hours or High for 2–3 hours, until chicken is tender and reaches 165°F internally.
Breasts lean toward the shorter end; thighs can go longer.
Finish with Parmesan and basil: Stir in Parmesan until melted and smooth. Taste and adjust salt and pepper. Add chopped basil.
Thicken if needed: If you’d like a thicker sauce, whisk 1–2 teaspoons cornstarch with 1 tablespoon cold water.
Stir into the sauce, cover, and cook on High for 10–15 minutes.
Serve: Slice or shred the chicken and spoon the sauce over pasta, rice, or potatoes. Garnish with extra basil and Parmesan.