Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and halve or quarter them so they’re all roughly the same size.
Put the potatoes in a pan with boiling water. Cover and simmer for 12-15 mins till the potatoes are just tender – it should be easy to insert a knife tip or skewer.
While the potatoes cook, add 1 tbsp oil to a roasting tin. Slide the tin into the oven to heat up. Peel and finely chop the garlic.
Drain the potatoes well. Tip them back into the pan. Gently crush them with a fork or potato masher so they’re lightly smashed, not completely crushed.
Tip the smashed potatoes into the hot roasting tin. Add the garlic and the thyme sprigs. Season. Give everything a stir. Roast in the oven for 30-35 mins till the potatoes are golden and crisp.
Trim the tops and tails off the runner beans. If they have a thread, run a vegetable peeler down each side to remove the string. Finely slice the beans. Set aside.
Rub 1 tsp oil into each steak. Season. Heat a griddle or frying pan till it’s smoking hot. Fry the steaks for 1-3 mins each side, depending on how well done you like them. Transfer to a plate and cover with foil. Set aside to rest for 5 mins.
Simmer the beans in a pan of boiling water, uncovered, for 3-4 mins till they’re just tender. Drain. Serve the steak and beans with the potatoes and a dab of mustard on the side.