Remove the whole chicken from packaging, rinse if desired, and pat dry thoroughly with paper towels.
Season the inside cavity and the outside of the chicken generously with salt and pepper.
Sprinkle 1 teaspoon dried thyme evenly over the chicken.
Pour 1/4 cup broth into the bottom of the slow cooker.
Place the chicken in the slow cooker breast-side up.
Cover and cook on LOW for about 8 hours; do not lift the lid while cooking.
When cooking is complete, the chicken should be tender; for crisper skin (optional), carefully transfer the chicken to an oven-safe skillet.
If using the broiler, brush melted butter over the skin and broil 3–5 minutes until golden and crisp, watching closely to avoid burning.
Garnish with lemon slices and rosemary if desired, then carve and serve.