Prepare the horseradish sauce first so the flavors can meld together. In a small bowl, whisk together light mayonnaise, reduced-fat sour cream, horseradish, Dijon mustard, lemon juice, garlic powder, salt, and pepper until smooth.
Pat the flank steak dry with paper towels. Sprinkle with garlic powder, onion powder, salt, and pepper.
Heat a grill pan or outdoor grill over medium-high heat. Cook 4-6 minutes per side, depending on the thickness and preferred doneness.
Let the steak rest for 5-10 minutes before slicing thinly against the grain.
Drizzle sauce over the steak or serve on the side for dipping.