Heat broiler to high. Rub one side of the baguette slices with the cut sides of the garlic and brush with 1 tablespoon of the olive oil. Broil on both sides until crisp and golden, about 1 minute per side. Set aside. In a large bowl, combine the remaining tablespoon of oil, tuna, capers, grapes, thyme, black pepper, and cayenne. Divide mixture between baguette slices and serve.