Prepare the Onions: Slice the onion into thin strands about 1/8-inch thick. The strands can vary in length but should be no shorter than 1 inch, with 2 inches being an ideal length.
Create the Soaking Mixture: In a large bowl, combine oat milk, lemon juice, and salt. Mix well.
Soak the Onions: Add the sliced onions to the oat milk mixture. Let them soak for at least 30 minutes, or up to an hour or more. The lemon juice will reduce the onion’s sharpness while the onions infuse the oat milk with flavor.
Coat the Onions: Add starch powder to the bowl with the soaked onions. Stir until a thin batter forms, lightly coating each onion strand. If the mixture seems too runny, add additional starch, one teaspoon at a time, until the batter thickens slightly.
Heat the Oil: Heat frying oil in a deep pot or fryer to 350°F.
Fry the Onions: Carefully add small clumps of the coated onions to the hot oil. Fry in batches for about 3 minutes, or until golden brown. Avoid overcrowding the pot to ensure the onions fry up light and crispy.
Drain and Cool: Transfer the fried onions to a paper towel-lined plate to drain excess oil.