Heat a grill pan coated with cooking spray over medium-high heat.
Combine salt, basil, and pepper in a small bowl. Sprinkle half of basil mixture over tomato and onion; rub lamb with remaining basil mixture. Add tomato, onion, and lamb to pan. Cook vegetables 2 minutes on each side or until lightly browned. Cook lamb 8 minutes on each side or until desired degree of doneness.