Prep the Potatoes: Place the cubed potatoes in a bowl of cold water and let them soak for at least 30 minutes, or up to 2 hours. Drain thoroughly and pat dry with paper towels.

Sear the Beef: Pat the beef cubes dry with paper towels. Season generously with salt and pepper. Heat 2 tablespoons of olive oil in a large cast-iron skillet over high heat until smoking hot. Add the beef in a single layer, being careful not to overcrowd the pan. Sear for 2-3 minutes per side, until browned. Remove the beef from the skillet and set aside.

Cook the Potatoes: Add the remaining 2 tablespoons of olive oil to the skillet. Add the potatoes in a single layer. Season with salt, pepper, smoked paprika, and red pepper flakes (if using). Cook, stirring occasionally, for 15-20 minutes, or until the potatoes are tender and browned on all sides.

Make the Garlic Butter: Reduce the heat to medium. Add 2 tablespoons of butter to the skillet. Once melted, add the minced garlic and thyme. Cook for 1-2 minutes, until fragrant, being careful not to burn the garlic.

Combine and Finish: Add the seared beef back to the skillet with the potatoes. Add the remaining 2 tablespoons of butter. Toss everything together to coat in the garlic butter. Cook for another 2-3 minutes, or until the beef is heated through and the butter is melted and bubbling.

Garnish and Serve: Garnish with fresh parsley and serve immediately.