In a mixing bowl, blend the sourdough starter with the egg and the tablespoon of oil until evenly mixed.
Incorporate the sugar, gingerbread spice, baking powder, baking soda, salt, and approximately 1¾ cups of the flour into the mixture, stirring gently until just combined. Avoid overmixing to maintain a soft, workable dough; add extra flour only as needed.
Roll the dough on a floured surface to a thickness of about ½ inch. Using a donut cutter, biscuit cutter, or glass (with a knife for the center), shape the donuts.
Arrange the cut donuts on a baking sheet and allow them to rise for 60 to 90 minutes. For quicker rising, preheat the oven to its lowest temperature, switch it off, and place the sheet inside.
To prepare in advance, let the donuts rise for 1 hour, then refrigerate. Prior to frying, bring them back to room temperature while heating the oil.
In the absence of a deep fryer, use a Dutch oven or similar pot with high sides and add 1 to 2 quarts of oil, ensuring it reaches at least one inch deep. Heat the oil to 375℉.
Line a plate with paper towels and place it near your cooking station. Fry a few donuts at a time for about 2 to 3 minutes per side until golden. Remove and drain on the paper towels, then transfer to a cooling rack or plate for optional topping application.