Certainly. They're cold-hardy veggies. A cold frame doesn't let in rain or snow, so keep an eye on the flat moisture and you could also keep the lid ajar so the air inside doesn't overheat.

why not winter sow them? I winter sown quite a few of the winter veggies couple weeks ago, they started sprouting after 10 to 15 days. I got better result with winter veggies than flower seeds in winter sowing.

Here is a link that might be useful: vegetable garden tips

Thank you, Trudi & Sara for answering.
Wouldn't planting in the cold frame act the same as winter sowing? I did start some onion seeds today. nothing else. I planted them in flats that I had saved from last year. I put newspaper on the bottom of the flat to hold the dirt. Suppose to snow again.. I am so wary about winter sowing but I have talked to others on here that have. And had success.