how do you blanch onions?

Answer 1

pearl onions
plunge into boiling water for a couple minutes.
Take out and place in cold water to cool them off thus stopping the cooking process.

For larger onions, it takes longer about 3 to 7 minutes.

Answer 2

Same way you blanch anything. Boil your water, place onion in it for 5-10 seconds, drain and place in bath of ice water (ice is actually there, not just cold). The ice helps stop the cooking process immediately. Drain and freeze or whatever you are doing with it.