Combine first 3 ingredients in a medium bowl, and stir well. Add tuna, 1/4 cup breadcrumbs, and green onions; stir well. Divide mixture into 4 equal portions, shaping each into a 4-inch patty. Press the remaining breadcrumbs evenly onto both sides of patties.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add patties; cover and cook 3 minutes. Carefully turn patties over; cook, uncovered, for 3 minutes or until patties are golden.
Spread mayonnaise evenly on the bun tops and bottoms. Place lettuce, tomato, onion, and patties on bottom halves of buns; place tops on sandwiches.