Microwave 1/2 tub Knorr® Homestyle Stock - Beef in small microwave-safe bowl at HIGH 30 seconds or until softened. Stir in 1 tablespoon olive oil and 1/2 teaspoon chili powder with wire whisk until smooth.
Brush steak, on both sides, with stock mixture. Refrigerate until ready to cook.
Cook onion with red pepper in 12-inch nonstick skillet over medium-high heat, stirring occasionally, until deep golden brown, about 15 minutes, reducing heat as needed to prevent scorching. Reduce heat to low. Stir in remaining 1/2 teaspoon chili powder, 1/2 tub Stock and 1/4 cup of water until Stock is melted. Stir in Mayonnaise and cheese until cheese is melted.
Meanwhile, grill or broil steak until desired doneness. Let stand 5 minutes, then thinly slice and arrange on serving platter. Top with onion mixture.