Slice zucchini in half crosswise; then slice into 1/4-inch lengthwise slices.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add half of the zucchini; cook 5 minutes. Turn slices over, sprinkle with half of the herbs and pepper; cook 5 minutes or until tender. Repeat with remaining zucchini, herbs, and pepper. Divide zucchini slices evenly among toast slices.
Combine tuna, mayonnaise, and onion in a small bowl, stir well. Spread 1/2 cup tuna mixture over each zucchini-topped bread slice; top each with 4 tomato slices and 1 cheese slice.
Place sandwiches on a baking sheet; broil 2 minutes or until cheese melts.