Fava beans might work. They are fairly cold hardy.

The main thing you need to consider is why you are planting a cover crop. There are multiple reasons for doing this:

1. Breaking up tough clay soil

2. Adding nitrogen to the soil

3. Erosion prevention

4. Attracting beneficial native insects and others

The choice of cover crop really depends upon your main reason for planting it. For instance, mustard is often used to break up tough soil given its deep, penetrating roots. Whereas, nitrogen fixing plants such as legumes are often used to add nitrogen to the soil. Grasses are often used for erosion prevention. There are lots of options for winter cover crops and they are super easy to maintain given the natural rains in the winter. I have bought seeds from Peaceful Valley before and have been pleased. They have a lot of options on their website. Of note, cover crops have the potential to steal significant nitrogen from the soil on a temporary basis so adding back fertilizer or compost is a very important part of building a healthy soil environment. There is even some controversy about nitrogen building cover crops actually stealing nitrogen from the soil in the short term. In the spring you can mow the crop and then cover it with mulch to try to seal in the nutrients. It's all a lot of work, but will be beneficial for your garden in the end.

-Brett