Melt the butter in a skillet over medium heat. Add onions, garlic, carrots, and celery, and cook until soft and fragrant.
Transfer the cooked vegetables into your crockpot. Add the shredded chicken, broth, milk, potatoes, thyme, salt, and pepper.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours until the potatoes are tender.
If you prefer a thicker soup, whisk flour with a little milk and stir it into the crockpot about 30 minutes before serving.
Add frozen peas near the end so they stay bright and fresh.
Garnish with fresh parsley before serving and enjoy with a slice of crusty bread.