Wash, peel and grate roughly the sweet potato (food processor or box grater).
Squeeze the water out of the grated sweet potato, using some paper towels or a kitchen towel. Don't skip this step, it will help them to stay compact. You should squeeze out approx. 1/2 cup of water.Transfer it into a medium bowl. Chop finely the red onion.
Stir together the grated sweet potato and red onion (and cornstarch, this is optional). Add cumin, salt and pepper to taste.
Heat some coconut or avocado oil in a non sticky skillet over medium-high heat.
Place a small handful of sweet potato into the skillet (approx. 1/4 cupful). Press firmly each batch with a spatula (to form a disc of approx. 10 cm like patties).
Cook for 4-5 minutes or so (or until the edge are turning brown).
Try to flip them delicately and cook the other side 3-4 more minutes (or toss if it's a bit messy). Repeat if you still have some sweet potato mixture left.