Basil & Garlic Slow Cooker Roasted Chicken

(5-6 lb) whole chicken- giblets removed, rinsed & patted dry

unsalted butter, room temperature

large handful of fresh basil (about 20-30 leaves) + 1 whole sprig of basil

garlic, minced

garlic, peeled and smashed

Salt & pepper,

baby carrots

large onion, chopped