To make Biscuits: COMBINE warm water and yeast in a small bowl. Set aside to soften. Stir together 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 mixture to dry ingredients; stir in buttermilk. 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. To make Filling: MELT 2 tablespoons butter in a large pot over medium-high heat. Add onion and cook about 5 minutes, until soft. Stir in chicken, vegetables, broth, garlic salt and pepper. Bring to a boil. COMBINE milk and corn starch in a small bowl. Stir into chicken mixture. Stirring constantly, bring mixture to a boil. Boil 1 minute. Spoon chicken mixture into a 2- or 3-quart casserole. TOP with the risen biscuits. Melt remaining 1 tablespoon butter and drizzle over biscuits. Place casserole on baking sheet (to catch any boil-overs in the oven). BAKE in 425°F preheated oven for 15 to 20 minutes, until filling is bubbly and biscuits are lightly browned.