Press zucchini between paper towels to remove excess moisture. Combine zucchini, breadcrumbs, and next 9 ingredients in a large bowl, mixing well.
Shape mixture into 8 (3-inch) patties. Coat a nonstick skillet with cooking spray; place over medium-high heat until hot. Cook patties 3 minutes on each side or until browned.
Combine mustard and mayonnaise; spread evenly on one side of 4 bread slices. Place 1 lettuce leaf and 2 tomato slices on coated side of each bread slice. Top each with 2 vegetable patties and a remaining bread slice. Serve immediately.