1. Halve lemon and place inside chicken cavity along with rosemary and garlic. Fold wings under and tie legs together. Rub chicken all over with 2 Tbsp. butter and sprinkle inside and out with salt and pepper.
2. Place chicken, breast side up, on a small rack inside slow cooker. Cover and cook on high until an instant-read thermometer inserted into thigh registers 180ºF, 4 to 4 1/2 hours.
3. Preheat broiler to high. Melt remaining 1 Tbsp. butter. Transfer chicken, breast side up, to a foil-lined baking sheet. Brush chicken with melted butter and broil to brown skin, 2 to 3 minutes. Let chicken rest for 10 minutes on a cutting board before carving and serving.