Preheat the oven to 200 degrees. Heat 1/4 cup olive oil in a pan. Add the onion and the garlic and cook until they become transparent, about 3 minutes.
Add tomatoes and mirin (or vinegar and 1 tablespoon of sugar). Boil while stirring to moderate heat, until the mixture becomes almost dry or about 15 minutes.
Heat 2 tablespoons of olive oil in a large pan. Season the steaks with salt and pepper. Sauté until they get a nice golden-brown color, about 1.5 minutes on each side. Transfer them to a baking pan and roast them for about 5 minutes in the preheated oven.
Add the remaining 4 tablespoons of olive oil to the pan. Chop the white part of the leek. Add the leeks to the hot olive oil and fry until crisp.