Position a rack in the upper third of the oven; preheat to 400 degrees . Line a baking sheet with foil; coat with cooking spray.
Combine the cornflakes, pork, onion, egg, milk, tomatillo, cheddar cheese, onion powder, 1 tsp. salt and 1/2 tsp. pepper. Shape into a 5-by-8-inch rectangle on the sheet. Cover and freeze loaf until firm, 1 hour.
Slice 3/4 inch thick; coat with cornflakes on all sides. In nonstick skillet, fry slices in 3 tbsp. vegetable oil over medium-low heat, turning once, until golden and cooked through, about 8 minutes.
