Halve lemon and place inside cavity of chicken along with rosemary, garlic and 2 Tbsp. butter. Fold wings under chicken and tie legs together with string. Sprinkle with salt and pepper.
Place chicken, breast side up, on a small rack inside slow cooker. Cook on high until an instant-read thermometer inserted into thigh registers 180ºF, 4 to 4 1/2 hours.
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 rest 10 minutes on a cutting board before carving and serving.