Heat oven to 400°F. In large skillet, heat oil over medium-high heat. Add onion and cook until soft. Add garlic, chili powder, beef, and salt. Cook, stirring frequently, until beef is browned. Drain meat. Stir in vegetables and salsa.
Cut each piecrust into three triangles. Place crusts on large cookie sheet. Divide mixture between triangles. Sprinkle cheese over meat. Fold crusts over, seal edges, and cut slits in each. Bake 25 minutes.