Melt butter in large nonstick skillet over medium heat. Add onions; cook 5 to 6 minutes or until crisp-tender, stirring occasionally. Season with salt and pepper, as desired. Remove from skillet; set aside.
Combine brown sugar and chili powder; press evenly onto beef steaks. Place steaks in same skillet over medium heat; cook 14 to 15 minutes for medium rare (145°F) doneness, turning occasionally. (Do not overcook.) Remove steaks; keep warm.
Return onions to skillet. Cook and stir 1 to 2 minutes or until heated through and browned bits attached to bottom of skillet are dissolved.
Carve steaks into thin slices; season with salt and pepper, as desired. Serve with onions.
Nutrition information per serving: 241 Calories; 81 kcal Calories from fat; 9 g Total Fat; 4 g Saturated Fat; 3 g Monounsaturated Fat; 73 mg Cholesterol; 79 mg Sodium; 16 g Total Carbohydrate; 1.3 g Dietary Fiber; 24 g Protein; 2.6 mg Iron; 4.7 mg Niacin; 0.6 mg Vitamin B6; 2.8 mcg Vitamin B12; 6 mg Zinc; 33.7 mcg Selenium;