Biscuits:

1. Combine flour, sugar, baking powder and salt in a large bowl. Cut in shortening using a pastry blender OR two knives until mixture resembles coarse crumbs. Add yeast to warm water. Stir to dissolve. Add yeast mixture and buttermilk to dry ingredients; stir until combined.
2. Place dough on a lightly floured surface. Knead 10 to 15 times; form into a ball. Pat dough to 3/4-inch thickness. Cut 10 to 12 biscuits using a 2-inch biscuit cutter (or use a straight sided drinking glass); place on ungreased baking sheet. Cover; let rise in a warm, draft-free place until doubled in size, about 30 minutes. Make filling while dough is rising.

Filling:

1. Preheat oven to 400°F.
2. Melt 2 tablespoons butter in a large saucepan over medium-high heat. Add onion and cook about 5 minutes, until onion is soft. Stir in chicken, vegetables, broth, garlic, salt and pepper. Bring to a boil. Combine milk and corn starch in a measuring cup or small bowl. Stir into chicken mixture. Stirring constantly, bring mixture to a boil and boil 1 minute.
3. Spoon chicken mixture into a 2 or 3-quart casserole. Top with the risen biscuits. Melt remaining 1 tablespoon of butter and drizzle over biscuits. Place casserole on baking sheet (to catch any boil-overs in the oven).
4. Bake for 15 to 20 minutes, until filling is bubbly and biscuits are lightly browned.