Season steak, red peppers and onion, if desired, with salt and ground black pepper. Combine 2 tablespoons oil and chili powder; brush on steak and vegetables.
Grill or broil steak and vegetables, turning once, until steak is desired doneness and vegetables are tender. Let steak rest 10 minutes before thinly slicing. Slice peppers.
Serve sliced steak, vegetables and hot Knorr® Fiesta Sides™ - Mexican Rice in tortillas with your favorite fajita toppings.