Take the steaks out of the fridge. Let to rest for min 30 mins, so they reach room temperature.
Preheat the oven for 200C/180C fan.
Wash the potatoes under cold water, and scrub with a brush to remove any remaining dirt. Cut them into halves and then into 3-4 wedges of roughly the same size. Put the potatoes into a large mixing bowl, add 3 tsbp olive oil, salt and 1 teaspoon rosemary. Toss well, so that all wedges are covered in the oil and seasoning. Place on a baking tray and put on the middle shelf of the oven, preferably skin down (this way you won't have to turn them halfway through). Bake for 30-40 mins, until golden and crispy on top.
Prepare the salad: Wash and cut the tomatoes and cucumber into same-size bites. Wash the pepper, remove all seeds and the inner membrane and cut into small pieces. Put the vegetables in a bowl, add 1 teaspoon olive oil and season with black pepper. Mix thoroughly. Divide the salad into two portions and serve in small serving bowls.
Heat the olive oil in a frying (or griddle) pan on high heat.
Season the pan with a pinch of salt and black pepper.
Once the pan is hot enough (but not smoky!), add the tuna steaks. Sprinkle with salt and pepper on top.
Cook the tuna to the desired level, turning occasionally (it can be cooked blue, rare, medium or well-done, just like beef steaks). For example, ½ inch thick steak takes about 6 minutes to cook until it is cooked through yet still pink and juicy inside.
Set aside to rest for about 3 minutes, pour over some lemon juice and serve with potato wedges and salad.