01 - Whisk together sour cream, prepared horseradish, Dijon mustard, lemon juice, salt, and pepper in a small bowl until smooth and well combined. Cover and refrigerate until serving time to allow flavors to meld.
02 - Pat steak cubes completely dry with paper towels to ensure proper searing. Season all sides evenly with kosher salt and freshly ground black pepper, pressing gently to help the seasoning adhere.
03 - Heat olive oil in a large skillet over high heat until the surface shimmers and the oil just begins to smoke slightly, indicating proper temperature for searing.
04 - Add seasoned steak cubes in a single layer, working in batches if necessary to avoid overcrowding. Sear for 1-2 minutes per side until deeply browned and just cooked through to medium-rare. Transfer to a plate and tent loosely with foil.
05 - Reduce heat to medium. Add butter to the skillet and let melt completely. Add minced garlic and sauté for 30 seconds until fragrant and lightly golden, being careful not to burn.
06 - Return seared steak bites to the skillet and toss gently to coat evenly in the garlic butter for about 1 minute. Sprinkle with chopped parsley and serve immediately with the chilled horseradish sauce alongside.