In a bowl, combine ground pork, ground pork fat, sugar, garlic, soy sauce, vinegar, salt, and pepper.
Add cornstarch. Mix until well-distributed.
On an individual sheet of plastic film or wax paper, spoon about a heaping tablespoon of pork mixture and form into a log. Roll and wrap tightly.
Repeat until all the mixture is shaped and wrapped. Arrange sausages in a ziplock bag or airtight container and freeze.
To cook, remove wax paper. In a wide pan over medium heat, arrange skinless longganisa in a single layer and add enough water to cover sausages halfway.
Bring to a simmer, cover, and cook until most of the water is reduced and sausages are cooked through.
Add the oil and cook, turning as needed, until longganisa are lightly browned and caramelized.
Remove from pan and serve hot.