Cut bread in half lengthwise (save one half for another use). Brush bread with oil. Place cut side up on an ungreased baking sheet. Broil 4-6 in. from the heat for 2-3 minutes or until golden brown. Rub cut sides of garlic clove over bread; discard garlic.
In a large bowl, combine the mayonnaise, lemon juice, mustard, garlic powder and pepper. Stir in tuna and artichokes.
Arrange spinach over bread; top with tuna mixture, tomatoes and cheese. Broil for 1-2 minutes or until cheese is melted. Cut into six slices.