Defrost the fish as described below
Scrub the potatoes without removing the skin. Cook in water for 20 to 30 minutes, depending on their size.
Finely chop the rosemary. Melt butter with 1 tablespoon of oil. Stir-fry the potatoes and the rosemary.
Grate the lemon to obtain its zest and finely chop the small chili.
Place the Tuna steaks on the barbecue, plancha or a hot grill. Sear on both sides. Add salt and pepper.
Immediately serve with the accompaniments.