Combine tuna, mayonnaise, mustard, lemon zest and juice, parsley, capers, remaining 1/2 tsp. salt, black pepper, and fennel fronds in a bowl; stir well. Spoon tuna mixture evenly onto bread slices. Top with grated cheese. Broil on HIGH 2 to 3 minutes, watching closely to not burn, until cheese is melted and crust is golden.