How to Make It
In a deep skillet, combine the celery, carrot, onion, vinegar, a pinch of salt and cover with 1 1/2 inches of water. Turn the heat to moderately high, cover and boil for 10 minutes. Add the tuna steak; when the water in the skillet returns to a boil, adjust the heat to moderately low so that the water simmers gently. Cover and cook the tuna until it is just pink in the center, 5 to 8 minutes.
Meanwhile, in a bowl, combine the garlic, anchovies and capers with the lemon juice, olive oil and mustard. Season with salt and pepper and beat with a fork to combine.
Remove the tuna from the skillet and pat dry. Slice it 1/2 inch thick. Choose a deep glass or ceramic dish that will hold the tuna in a single layer. Lightly spread some of the sauce over the bottom of the dish. Add the tuna slices, laying them flat, and cover with the remaining sauce, spreading it evenly. Cover the dish with plastic wrap and let stand at room temperature for 6 hours, then refrigerate overnight. Bring to cool room temperature before serving.