Preheat your oven to 425°F (220°C).
In a large pot, melt butter over medium heat.
Add onion, cooking until translucent.
Stir in flour and cook for 1 minute.
Gradually whisk in chicken broth and milk.
Bring the mixture to a simmer until thickened.
Add the chicken, carrots, peas, potatoes, salt, pepper, and thyme.
Pour the filling into a baking dish.
Cut biscuits and place them on top of the filling.
Bake for 25-30 minutes or until biscuits are golden brown.