Preheat the oven to 200C/180C Fan/Gas 6.
Boil the potato cubes in a pan of salted water for 3-4 minutes. Drain well and set aside.
Meanwhile, melt 50g/1¾oz of the butter in a frying pan over a medium heat. Add the shallot, garlic and leeks and fry for 2-3 minutes, or until softened.
Add the drained potato cubes and stir carefully to combine. Pour in the white wine and bring the mixture to the boil. Reduce the heat until the mixture is simmering, then stir in the cream and thyme leaves and continue to simmer for 2-3 minutes. Season, to taste, with salt and freshly ground black pepper. Transfer the potato mixture to an ovenproof dish.
Heat a further 50g/1¾oz butter in a separate frying pan over a medium to high heat. When the butter is foaming, add the brioche cubes and fry for 1-2 minutes on all sides, or until golden-brown and crisp. Scatter the brioche cubes over the potato mixture, then sprinkle over the grated Gruyère. Bake in the oven for 5-10 minutes, or until the cheese is golden-brown and the mixture is bubbling.
Meanwhile, heat a griddle pan over a very high heat until searing hot. Rub the steaks all over with the rapeseed oil, and brush the griddle pan with a little of the rapeseed oil, too.
Add the steaks to the pan and fry for 2 minutes, without moving them. Give the steaks a quarter-turn and fry for another 1 minute. Season the uncooked side with salt and freshly ground black pepper.
Turn the steaks over and fry for 1-2 minutes on the other side (for medium), or longer until cooked to your liking. Transfer the steaks to a warmed plate and set aside to rest for 5 minutes.
Melt the remaining butter in the pan used to fry the brioche, then brush it over the rested steaks, spooning any liquid that has been released from the meat back over the steaks. Season once more, to taste.
To serve, place a steak on each plate and spoon over the meat juices. Spoon the potato and leek gratin alongside.