This is how I put onions up.
This method can be used on homegrown and store bought onions. If the onions have a strong scent, you may want to double package them before freezing and/or pack into another freezer container.
Green onions - Remember to harvest only the green onion tops. Clean all the dead or dry stalks from onions; trim some of the green tips away from the stalks if they are dry, brown or soft. Rinse the green onions under cool running water. Dice cleaned onion and flash freeze same as bulb onions. Store in a food/freezer safe container.
Bulb onions - remove outer peel from onion bulbs and discard. Slice off the neck and root ends. Bulb onions should be firm and free of imperfections completely through the center. Rinse the onion bulb under cool running water and pat dry. On a cutting board, dice the onion and spread in a single layer onto a cookie sheet; lay in the freezer until frozen.
Remove frozen onions from cookie sheet and store in a food/freezer-safe container.
Be sure to label and date all foods you put up.