01 - Whisk together sour cream, horseradish, Dijon mustard, lemon juice, salt, and pepper in a small bowl. Cover and refrigerate until ready to serve.
02 - Pat steak cubes dry with paper towels. Season evenly with kosher salt and black pepper on all sides.
03 - Heat olive oil in a large skillet over high heat until shimmering.
04 - Add steak cubes in a single layer, working in batches if necessary. Sear for 1–2 minutes per side until browned and just cooked through. Remove steak to a plate and tent with foil.
05 - Reduce heat to medium. Add butter to the skillet and let melt. Add minced garlic and sauté for 30 seconds until fragrant.
06 - Return steak bites to the skillet and toss to coat in the garlic butter for about 1 minute. Sprinkle with chopped parsley and serve immediately with the chilled horseradish sauce on the side.