Slice the loaf in half lengthwise. Spread the goat cheese evenly over cut side of bottom half of loaf, and spread the olive paste evenly over goat cheese.
Arrange arugula, onion rings, tomato slices, basil, and chopped capers on top. Drizzle with vinegar and olive oil. Sprinkle with pepper. Replace with the top half of the loaf. Cut crosswise into 4 pieces.