Cut the potatoes into 1.5 cm (1/2 inch) cubes. Trim the steak and cut it into 2.5 cm (1 inch) cubes, pat dry and season with the salt and pepper. Mince the garlic.
Heat the oil in a large skillet with a lid over medium heat. Add the potatoes, cover and cook for 10 minutes, stirring twice.
Uncover, turn to medium-high and cook 8 minutes more, stirring now and then, until crisp and tender. Tip into a bowl.
Turn the heat to high. Add the steak cubes in one layer and sear without moving for 1 1/2 minutes, then turn and sear 1 1/2 minutes more, until browned and 63°C (145°F) inside for medium-rare.
Lower the heat to medium, add the butter, garlic and thyme and toss for 1 minute until the garlic is fragrant. Return the potatoes and toss to coat.
Steam the green beans in a covered pot over 2.5 cm (1 inch) of boiling water on high heat for 4 minutes.
Serve the steak and potatoes with the green beans, scattered with parsley.