Preheat oven to 400°F.
Melt 2 tablespoons butter in 3-quart saucepan over medium high heat,. Add chicken and onion; stirring frequently, cook 5 minutes, or until onion is translucent. Stir in vegetables, chicken broth, salt, pepper and thyme. Bring to a boil.
Blend milk and corn starch together in small measuring cup. Stir into chicken mixture. Bring mixture back to a boil, stirring constantly, and boil 1 minute.
Spoon chicken mixture into a 2-quart casserole dish. Top with biscuits; drizzle with 1 tablespoon of melted butter.
Bake at 400°F for 15 to 20 minutes or until filling is bubbly and biscuits are lightly browned.