Halve bread horizontally. Pull out the soft interior, to within 1-inch of the crust; discard interior or reserve for another use. Rub the inside of bread all over with garlic, then drizzle with oil and vinegar.
Cover the bottom half of the bread with onion and tomato slices. Top with anchovies and pepper strips. Evenly distribute tuna and basil.
Spread the top half of bread with minced olives, and place it olive-side down over filled bottom half. Wrap in plastic wrap. Place on a large plate, and top with a second plate. Weight with several heavy cans. Refrigerate for 2 to 3 hours. Just before serving, cut into 6 to 8 wedges or slices.