Cut onions into evenly thin slices and ensure they are almost of same thickness.
Sprinkle flour and cornstarch on the slices and toss them.
Heat oil in a deep pan till medium hot but not smoking hot (350°F).
Reduce the flame to medium. Add a few onion slices at a time and deep fry until crisp.
Transfer them to an absorbent paper. Fry all the onions in batches.
Cool them completely and store in an airtight jar.
Notes
Buttermilk soak (optional for French-style): Soak sliced onions in buttermilk for 10 to 15 minutes before coating for a crispier, more textured coating. Drain well before dredging.No thermometer: Dip a wooden spoon handle into the oil. Steady bubbles around the handle means the oil is ready.Seasoning ideas: Add garlic powder, paprika, cayenne, onion powder, or 1 tablespoon Cajun seasoning to the flour mixture.Gluten-free: Substitute a 1:1 GF flour blend for the all-purpose flour.Air fryer: Toss in a little oil, air fry at 375º F until crisp.Oven: Spread on parchment, spray with oil, bake at 400º F flipping once.Storage: Room temperature, airtight, 2 to 3 days. Refrigerator, up to 1 week. Freezer, up to 2 months.Re-crisping: Air fryer at 375º F for 1 to 2 minutes, or oven at 375º F for 3 to 5 minutes.