1In 12-inch nonstick skillet, melt butter over medium-high heat. Cook onion in butter 3 to 4 minutes, stirring frequently, until tender; push to side of skillet.
2Add beef steaks to skillet; sprinkle with salt and pepper. Cook 5 to 8 minutes, turning once, for medium doneness (160°F).
3Spread pesto on cut sides of buns. Layer steaks, cheese, onion and tomato in buns.