1. Mix chicken, eggs, onion, bread crumbs, parsley, parmesan cheese, salt, baking soda, pepper and oregano in large bowl.
2. Shape mixture into 4 round patties.
3. Heat butter and oil in large skillet over medium-low heat until butter melts.
4. Add patties and cook until golden, 4-5 minutes on each side.
5. Place 1 slice of cheese on each patty.
6. Cover skillet; cook until cheese melts, about 1 minute.
7. Spread each bun with 1 tablespoon mayonnaise.
8. Arrange lettuce leaf over bottom half of each bun.
9. Top with a patty, 1 or 2 slices tomato and top half of bun.
10. Serve hot.