Preheat oven to 400°F (200°C).
In a skillet, melt butter over medium heat. Add diced onion and cook until translucent.
Stir in flour and cook for 1-2 minutes. Gradually add chicken broth and heavy cream, stirring constantly until thickened.
Add shredded chicken, peas and carrots, salt, and pepper. Mix well and remove from heat.
Pour the filling into a baking dish.
In a mixing bowl, combine flour, baking powder, and salt for the biscuit crust. Cut in cold butter until the mixture resembles coarse crumbs. Stir in milk until just combined.
Drop biscuit dough by spoonfuls over the filling in the baking dish.
Bake in preheated oven for 25-30 minutes, or until the biscuit crust is golden brown.