Melt 1 Tbsp butter in a large nonstick skillet. Over medium heat, cook onions until softened and translucent. Transfer to slow cooker.
Add frozen peas and frozen crinkle cut carrots to slow cooker.
Add frozen chicken breast to slow cooker.
Add chicken broth, 1 cup water, and can of cream of chicken to slow cooker. Add remaining 2 Tbsp butter and thyme. Cover and cook until chicken is tender, about 4-6 hours on low.
Once chicken is cooked, remove chicken and transfer to cutting board. Let cool slightly and then shred into bite-sized chunks using two forks. Return shredded chicken to slow cooker. Stir to distribute among other ingredients. Season with salt and pepper, to taste. Cover slow cooker as you prepare dumplings.