1. Cut tuna in half horizontally. Each half should measure 2 by 2 by 3/4 inches. Cut each piece in half. Cut each of the four strips into quarters (you will have sixteen small cubes). Cover tuna with plastic wrap, and refrigerate.

2. In a small nonstick saute pan, heat grapeseed oil over medium-high heat. Add garlic, and saute until golden brown. Remove saute pan from heat, and add shallot and rosemary. Season with salt and pepper. Add sherry-wine vinegar and olive oil; stir to combine. Transfer to a shallow dish, and refrigerate for 2 to 3 minutes. Add anchovies, and stir to coat evenly. Refrigerate for at least 3 hours and preferably overnight.

3. Wrap each cube of tuna with an anchovy, and skewer where the tail and the head of the anchovy meet. Place in the marinade, and turn to coat. Transfer to a platter, and serve with a drop of balsamic vinegar next to each skewer and a few grains of gray salt sprinkled over each piece of fish.