1. Combine ground round, 2 tablespoons onion, and 2 tablespoons chili powder; shape into 6 (1/2-inch-thick) patties.
2. Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add patties; cook 4 minutes on each side or until done.
3. While burgers cook, combine remaining onion, tomato, 1 teaspoon chili powder, and lime juice. Place patties on bottom halves of buns; spoon salsa over patties, and cover with bun tops.