To the slow cooker add the onion, carrots, celery and garlic.
Stir in turkey, cream of chicken soup, cream of celery soup, chicken broth, poultry seasoning, salt, and black pepper. Mix well to combine.
Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until vegetables are tender and the broth is creamy.
About 30 minutes before serving, stir in frozen peas (if using).
In a bowl, mix Bisquick, milk, and parsley until a soft dough forms.
Drop spoonfuls of dough on top of the hot soup. Cover and cook on HIGH for 1 additional hour, or until dumplings are puffed and cooked through.
Gently stir once to mix the dumplings slightly into the soup, garnish with fresh parsley, and serve warm.