Thaw, wash and pat dry the tuna steaks. Then pepper, salt and drizzle with lemon juice.
Cut the leek into fine rings and fry in a pan in butter until golden. Add the white wine and crème fraîche, season with salt, pepper and nutmeg and cover and simmer over a medium heat for 10 minutes.
Place the tuna on the vegetables and cook everything together for another 15 minutes. Sprinkle with a little mint to decorate and refine.
It goes well with fresh baguette or green ribbon noodles or rice.