2 lb sweet potatoes, peeled and cut into 1-inch cubes 1 lb extra-lean ground sirloin 1/2 cup frozen corn 1/2 cup frozen peas 4 extra-large egg whites, divided 1/4 teaspoon sea salt 2 slices whole-grain bread 1 tablespoon olive oil, divided

1. Fill a large pot halfway with water and bring to a boil. Add potatoes and reudce heat to a simmer. Cover and cook until just tender, about 10 minutes. Drain.
2. Meanwhile, in a large bowl, combine sirloin, corn, peas, 1 egg white and salt. Mix thouroughly with your hands and shape into 8 3 1/2-inch wide x 1-inch thick patties.
3. In a food processor, process bread into fine crumbs. Transfer potatoes to a separate large bowl and use a potato masher to mash well. With an electric hand mixer, beat remaining 3 egg whites into potatoes on high speed until light and creamy, about 1 minute. Stir in bread crumbs. Form into 12 3 1/2-inch-wide patties and set aside.
4. Heat a large nonstick skillet on medium. Working in batches, add beef patties and cook, shaking pan occasionally, for 3 minutes. Flip patties and press down firmly on tops with a heat-proof spatula. Cook for 2 minutes. Press firmly on tops again, and cook for 2 more minutes, until cooked through. Transfer to a large plate and cover loosely with foil.
5. Heat a separate large nonstick skillet on medium. Working in three batches, add oil, then potatoe patties, and cook, shaking skillet occasionally, for 4 minutes. Flip patties and cook for 3-4 more minutes, until golden brown. Transfer to a separate large plate.
6. Assemble burgers: Arrange 1 potato patty on a clean work surface and top with 2 each beef and potato patties, alternating between each to end with potato. Repeat with remaining potato and beef patties.