Prep the base: Add potatoes, onion, salt, pepper, garlic, rosemary, and cayenne pepper to the bottom of your crockpot.
Add chicken and corn: Place chicken breasts or thighs on top of the vegetables. Pour in corn and broth.
Cook: Cover and cook on low for 6–7 hours or high for 3–4 hours, until chicken is tender and cooked through.
Shred chicken: Remove chicken from the slow cooker, shred with two forks, and return to the pot.
Thicken chowder: In a small bowl, whisk milk and cornstarch until smooth. Stir mixture into the chowder. Add chopped bell pepper. Cover and cook for another 30 minutes, until chowder thickens.
Finish creamy base: Stir in Greek yogurt until smooth and creamy. Taste and adjust seasoning with additional salt, pepper, or broth if needed.
Serve: Ladle into bowls and garnish with green onions if desired. Serve with crusty bread or biscuits.