Cover the bottom of the slow cooker with about half of the leeks and onions. Add half the garlic cloves. Add the chicken and sprinkle with salt, pepper, thyme, and paprika. Top with remaining leeks, onions, and garlic. Pour into the chicken broth and add bay leaf. Stir to combine.
Cover and cook on high for 4 hours or on low for 7-8 hours. Remove the chicken from the slow cooker. Add the butter and stir until melted. Mix the cornstarch with water. Add to the slow cooker and stir. Let sit for 5-10 minutes until sauce thickens.