Pat the steak cubes very dry with paper towels — moisture is the enemy of browning. Season with 3/4 teaspoon of the salt and 1/2 teaspoon of the pepper. Trim the green beans and cut any long ones in half.

Bring a medium pot of salted water to a boil. Add the green beans and cook 2–3 minutes until bright green and crisp-tender. Drain and set aside. This par-cook means they only need a finishing sear later.

Heat 2 tablespoons of the olive oil in the large skillet over high heat until just smoking. Add half the steak cubes in a single layer — do not crowd them — and sear undisturbed for 2 minutes, then flip and sear 1–2 minutes more until deeply browned on the outside. Transfer to a plate. Repeat with the remaining oil and steak.

Reduce the heat to medium-high. Add the blanched beans to the skillet with the remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring occasionally, 2–3 minutes until they pick up blistered spots. Transfer to the plate with the steak.

Reduce the heat to medium-low. Melt the butter in the skillet, then add the garlic, thyme, and red pepper flakes if using. Cook, stirring constantly, 1–2 minutes until the garlic is fragrant and pale gold at the edges — not brown, which tastes bitter.

Return the steak and beans (plus any juices from the plate) to the skillet and toss in the foaming garlic butter until everything glistens, 1–2 minutes. The residual heat finishes the steak while the butter coats every bite. Shower with parsley and serve immediately with lemon wedges.