Method
Pat the beef cubes completely dry with paper towels and season evenly with kosher salt and black pepper.
Heat olive oil in a large skillet over medium-high heat until shimmering but not smoking.
Add beef cubes in a single layer without overcrowding and sear undisturbed for 2-3 minutes until a golden-brown crust forms on the bottom.
Flip beef cubes and continue cooking for another 2-3 minutes until browned on all sides and internal temperature reaches 135°F for medium-rare.
Reduce heat to low and push beef to one side of the skillet, then add butter and minced garlic to the empty space.
Stir garlic constantly for 30-45 seconds until fragrant but not browned, then toss everything together to coat the beef evenly.
Remove from heat and immediately stir in chopped parsley and optional red pepper flakes until well combined.
Transfer to a serving plate and drizzle with any remaining garlic butter from the skillet.