Onions are tricky to cut, especially if the goal is perfectly thin slices. To make things easier, swap a knife for a vegetable peeler, which works well thanks to its unique shape.
Featuring a slotted space between two blades, this small tool fits perfectly in your hand. The Y-shaped type is best for cutting onions, but other varieties will also work.
Once the onion is stable, take the Y-shaped vegetable peeler and slice back and forth over the onion's top end. You'll soon have a small pile of fine and evenly sliced onions.