Adjust the oven rack to upper-middle position and heat oven to 450 degrees.
Bake rolls, cut side up, on wire rack inside rimmed baking sheet until golden and crisp, about 5 minutes.
Combine cornflakes, onion, milk, 4 tablespoons ketchup, Worcestershire sauce, thyme, garlic powder, egg, 1/2 teaspoon salt, and 1 teaspoon pepper in large bowl. Add meat and knead gently until well combined.
Top toasted rolls evenly with meat mixture, spreading meat to edges of rolls. Brush top of meat mixture evenly with remaining ketchup and bake until meat registers 160 degrees, 20-25 minutes. Top burgers with cheese and continue to bake until cheese melts, 3-4 minutes. Let rest 5 minutes. Serve.