Heat the oil in a skillet over medium heat. Cook the onion in the hot oil until lightly browned; set aside and allow to cool.
Combine the crackers and cornflakes in a food processor; pulse until processed into crumbs; transfer to a large bowl. Add carrots, salt, pepper, and garlic powder to the crumbs. Mix in the eggs, onions, margarine, and water. Use your hands to mix until well integrated.
Pack lightly into uncooked turkey. Roast turkey as directed.