Cut onion into slices about 1/4-inch (1/2 centimeter) wide. Separate the layers into individual rings.

Set up a breading station: Place potato starch in one bowl; eggs, spices, and sugar in a second bowl (whisk together to combine); and ground walnuts in a third.

Dip each ring first into potato starch, then into the egg mixture, and then into the ground walnuts. Place on a platter or baking sheet. I recommend doing this all at once before you start frying, because the nut crust on these onion rings will taste really burnt if they cook too long. You don’t want to get distracted when you start frying!

Heat oil over medium heat. Fry a few rings at a time for about 30 seconds per side, then remove to paper towels to drain. Fry in batches to avoid overcrowding the pan.

Whisk all the ingredients together in a bowl until smooth. Serve alongside onion rings.