Preheat the oven to 400 degrees. Spray a large 9x13-inch baking dish with olive cooking spray.
Cook the potatoes in a large saucepan of well-salted water, bring to a boil, and cook for 5-7 minutes, until tender. Remove from heat, drain, and cool. Once cool, dice into small chunks.
Cook the vegetables by heating the olive oil in a large skillet over medium heat. Add the onion, mushrooms, carrots, and celery, and cook for 5 minutes, stirring often, or until slightly tender.
Add garlic and continue to cook, stirring frequently, for 45 seconds. Remove from heat and set aside.
Make the filling by heating the butter in a large skillet over medium-high heat. Add the flour and cook, stirring often, for 2 minutes.
Slowly whisk in the chicken broth and milk and simmer, stirring occasionally, for 7-10 minutes, until thickened.
Add sauteed vegetables, corn, peas, potatoes, and chicken to the mixture and stir; season with sea salt and freshly cracked black pepper to taste, and cook for 2-3 minutes. Pour into the baking dish.
Make the biscuits by combining the flour, baking powder, salt, sugar, and garlic powder until mixed thoroughly.
Stir in milk and melted butter just until the flour mixture is moistened.
Drop biscuit topping onto the hot mixture in the baking dish to form 16 small biscuits.
Place into the oven and bake for 30 minutes, or until biscuits are golden brown.