Put a large pan of salted water on to boil. Scrub the potatoes and roughly chop them into small, equal sized pieces.
When the water is boiling, add the potatoes. Simmer for 10-15 mins or till they're tender when pressed with a fork.
While the potatoes cook, peel and finely slice the red onion. Peel and grate or crush the garlic. Finely chop the rosemary leaves.
When the potatoes are cooked, drain them and tip them back into the pan. Roughly mash them so they break into large clumps (not a smooth mash).
Warm a frying pan for 1 min. Add 1 tbsp olive oil and the onion. Season with a pinch of salt and pepper. Cook over a medium heat for 5 mins, stirring often, till the onions have picked up a bit of colour.
Stir in the garlic. Cook and stir for 1 min. Add the potatoes to the pan with an extra 1/2 tbsp olive oil. Cook for 10-12 mins, stirring once or twice, so the potatoes crisp up and turn golden.
While the potatoes fry, put a griddle or frying pan on a high heat to warm up for 2-3 mins. Rub 1/2 tbsp oil and some salt and pepper into the steaks.
Add the steaks to the frying pan. Fry for 1-2 mins on each side, depending on how well done you like your steaks. Pop them on a plate and loosely cover with foil. Rest for 2 mins
Stir the rosemary into the new potato smash. Serve with the steaks and a handful of watercress.