Rub the steaks with olive oil, salt, pepper, and (optional) a sprinkle of thyme. Set to one side.
Put both types of potatoes and the garlic cloves in a large saucepan. Covver with cold water and bring to the boil. Simmer until tender, about 15-20 minutes, then drain. Mash, stir in the creme fraiche, season and keep warm.
Meanwhile, heat 1 tbsp oil in a frying pan over a low heat and cook the onion for 12-15 minutes until softened. Add the ivinegar and stir through. Set to one side.
Heat a griddle pan on a high heat. Add the steaks and cook for three or four minutes on each side (depending on thickness) for medium. Remove from the pan to rest for a few minutes.
Add 75ml water and the onions to the griddle pan and saute for a couple of minutes to finish off the onions.
Serve the ssteak with the mash and onions, and steamed greens / green veg.