Once an onion is cut, it starts releasing those famous sulfuric compounds that can permeate everything in your refrigerator.
The Butter Seal
Rub a tiny bit of butter or oil over the cut surface of the onion. This creates a thin barrier that prevents the gases from escaping and the onion from drying out.
The Airtight Glass
Plastic bags are surprisingly porous. For the best odor control, store the butter-sealed onion in a glass jar with a tight-fitting lid.