I stored a freshly cut onion with butter spread over the top and wrapped it with aluminum foil. It stayed fresh for three weeks. Is this a safe way to store onions
Onions have natural antibacterial and antiseptic properties. Butter spread is just fat that don't allow air to penetre thus not making it mold friendly.
I would say it's safe but there is no need to store cut onion. It's better to have onion head whole and cut as needed. If they start to sprout you cut the upper part put in the ground and have some fresh green onions.