Arrange the chicken thighs in a single layer in the slow cooker. Slice one lemon and set aside. Juice the remaining lemon and drizzle the juice over the chicken pieces. Sprinkle with the garlic, kosher salt, pepper, and oregano or basil. Top with the lemon slices and onions or shallots if using.
Cover and cook on HIGH for 3 to 4 hours.
Adjust a rack in the oven to the highest level. Heat the broiler to HIGH (500 F).
Transfer the chicken thighs to rack in a baking pan (arranged skin-side up) and place it under the broiler for about 4 to 6 minutes, or until the skin is golden brown and crispy.