Preheat oven to 375 degrees F.
In a large oven-proof pot, melt butter.
Add onion, carrots, and celery, cooking until softened, about 7-8 minutes.
Add peas and garlic and cook another 2-3 minutes.
Season with salt, pepper, and mustard powder.
During the time the vegetables are cooking, you can make the biscuits (directions below).
Once the vegetables are cooked, add the flour, stirring well to combine.
Cook for 1 minute, stirring often.
Slowly add chicken broth and milk, stirring until well combined.
Stir in chicken.
Over medium heat, bring to a simmer and let it thicken for about 5 minutes.