Prepare the Vegetables and Cheese: Peel and grate the potato using the large holes of a box grater. Immediately place the grated potato in a clean kitchen towel or cheesecloth and squeeze firmly to remove as much water as possible. This step is crucial — excess moisture prevents the fritters from crisping properly.
Grate the Onion: Grate the onion directly into a bowl. You can drain off some of the onion liquid if it seems excessive; a little moisture helps flavor the fritter but too much will thin the batter.
Add the Cheese: Grate your chosen cheeses and add them to the bowl. The blend of cheddar and Gruyère gives the fritters a wonderful balance of sharpness and nutty richness.
Season and Mix: Sprinkle in salt, pepper, and mustard powder. Add the flour and egg, then mix everything together with a fork or your hands until fully combined. The mixture should hold together easily when pressed. If it feels too wet, add an extra teaspoon of flour.
Heat the Oil: Pour enough oil into a large skillet to coat the bottom and heat it over medium heat. The oil is ready when a small drop of batter sizzles immediately upon contact. Too hot, and the fritters will brown before cooking through; too cool, and they’ll absorb excess oil.
Shape and Fry: Scoop a heaping spoonful of the mixture (about ¼ cup) and gently press it into a patty shape. Place it into the hot oil and flatten slightly with the back of a spoon. Fry for approximately 5 minutes per side, or until golden to deep brown and crisp around the edges. Adjust heat as needed to prevent burning.
Drain and Keep Warm: Once cooked, transfer the fritters to a plate lined with paper towels to drain off any excess oil. Keep them warm in a low oven while you cook the remaining batches.
