I have started to harden off my seedlings of Lettuce, Rapini AKA Broccoli Raab, and Onions outside. The easiest way for me to do this is to put them in a big cardboard box which protects them from the wind initially.
This is more than an art than true science, and requires patience. They will be out for about 3 hours today and I will increase the outdoor time and sun exposure each day. Based on the weather and how these seedlings look, it looks like it will take about a week or so before they are ready to be planted. I also decrease watering these plants during this time to toughen them up. I know it sounds heartless, but this is the way that I have found works for me.
If you have helpful tips for this chore, I would love to hear from you. Happy St. Patrick's Day too.