Place chicken breasts in the crockpot.
Add chopped onion and frozen mixed vegetables.
In a bowl, mix chicken broth, cream of chicken soup, garlic powder, thyme, salt, and pepper.
Pour the mixture over the chicken and vegetables in the crockpot.
Cover and cook on low for 6-8 hours or on high for 3-4 hours.
About 30 minutes before serving, prepare the dumplings by combining biscuit mix and milk.
Drop spoonfuls of the dumpling mixture on top of the chicken mixture.
Cover and cook for an additional 30 minutes on high.
Ensure the dumplings are cooked through.
Serve warm, garnished with fresh herbs if desired.