Heat oven to 350°F.
Pierce the sweet potato several times with a paring knife, then microwave on high until tender, 6 to 8 minutes.
Meanwhile, grind the oats in a food processor until they reach the consistency of whole wheat flour, approximately 3 minutes. Remove the oats from the food processor and set aside.
Combine the sweet potato pulp, sardines, egg, and peanut butter in the food processor. Pulse 10 to 12 times until the mixture forms a thick paste — there will be some lumps.
Add 2 cups of the ground oats and pulse until the mixture starts to come together, an additional 10 to 12 times. Similarly to pie dough, it will look dry, but should hold together when pressed.
Sprinkle your work surface with some of the remaining oats and roll the dough to 1/4-inch thickness. Cut into treats using a pizza wheel or cookie cutters; I used a 2-inch cutter.
Bake on a parchment paper-lined half sheet pan until treats appear dry and lightly browned on the bottom, 30 to 40 minutes. Turn off the oven, brush with the fish sauce, if desired, and let the treats dry in the oven for 1 hour.
Store in an airtight container for 1 week or freeze for up to 3 months.