Combine the cornflake crumbs, bread crumbs, and flour in a shallow dish; set aside. Grind the walleye fillets, or chop finely and place into a mixing bowl. Add the carrots, zucchini, onions, garlic, and eggs. Season with dry mustard, lemon juice, salt, and black pepper. Mix with your hands until evenly combined.
Form the walleye mixture into 1/3-cup cakes about 3/4-inch thick, and press into the cornflake mixture. Set the walleye cakes onto a baking sheet in a single layer -- do not stack.
Melt the butter in a large skillet over medium heat. Cook the walleye cakes in batches until firmed and golden brown on each side, about 8 minutes per side.
