Pat the steak cubes completely dry with paper towels. This is crucial for a good sear. Place them in a bowl and season generously on all sides with the kosher salt and black pepper. Let them sit at room temperature for 10-15 minutes while you prep the other ingredients.

Heat a large cast-iron or heavy-bottomed skillet over medium-high heat until very hot. Add the avocado oil and swirl to coat. Working in batches to avoid overcrowding, add the steak cubes in a single layer. Sear without moving for 2-3 minutes until a deep brown crust forms on the bottom. Flip and sear the other sides for another 1-2 minutes until browned but still medium-rare inside (about 125°F internal temp). Transfer the seared bites to a plate. Repeat with remaining steak if needed.

Reduce the heat to medium-low. Add 2 tablespoons of the butter to the same skillet. Once melted, add the minced garlic, thyme (if using), and red pepper flakes. Cook, stirring constantly, for about 1 minute until the garlic is fragrant and just starting to turn golden. Be careful not to burn it.

Add the remaining 4 tablespoons of butter to the skillet. Once melted and foamy, return all the seared steak bites and any accumulated juices back to the pan. Toss continuously for 1-2 minutes until the steak is coated in the sauce and heated through. Remove from heat, stir in the chopped parsley, and give it a final toss. Taste and adjust seasoning with more salt and pepper if needed.