Place 3 pounds boneless skinless chicken thighs in the bottom of a 5 to 6-quart slow cooker.

In a small bowl, stir together 2 teaspoons paprika, 1 ½ teaspoons salt, 1 teaspoon onion powder, 1 teaspoon garlic powder, ½ teaspoon black pepper, and ½ teaspoon crushed red pepper flakes. Sprinkle mixture evenly over the chicken.

Zest and juice 1 medium lemon. Add the lemon juice to the slow cooker and sprinkle the lemon zest and 1 tablespoon minced garlic over the top of the chicken thighs.

Dot the top of the chicken with 6 tablespoons salted butter, sliced into tablespoon-size pieces.

Cover and cook on low for 4 to 5 hours, or high for 3 hours, until chicken is tender and reaches an internal temperature of 172°F (74°C).

To garnish, slice 1 medium lemon to top the chicken and sprinkle with 1 tablespoon chopped fresh parsley, if desired. Serve hot, drizzled with the juices.