Parboil potato halves in salted boiling water for 6 to 8 minutes until fork-tender but firm. Drain thoroughly and let steam dry for 2 minutes.

Heat 2 tablespoons olive oil in a large 12-inch cast-iron skillet over medium-high heat. Place potatoes cut-side down, season with smoked paprika, salt, and pepper, and sear undisturbed for 4 to 5 minutes until deeply golden and crispy. Toss and cook 2 minutes more, then transfer to a warm side plate.

Pat sirloin cubes thoroughly bone-dry with paper towels to ensure a deep crust. Season immediately before searing with garlic powder, coarse salt, and cracked pepper.

Add avocado oil to the smoking-hot skillet over high heat. Add beef cubes in a single uncrowded layer (sear in two batches if needed). Sear untouched for 2 minutes to develop a dark caramelized crust. Flip with tongs and sear for 1.5 minutes more for succulent medium centers.

Lower heat to medium-low. Add cubed butter, minced garlic, fresh rosemary, and thyme. As the butter foams and turns fragrant, tilt skillet and vigorously spoon the garlic herb butter over the steak bites for 60 seconds.

Return crispy potatoes to the skillet, tossing gently for 1 minute so they soak up the warm garlic butter fond. Squeeze fresh lemon over top, garnish with fresh parsley, and serve sizzling hot.