I live in rainy Oregon, and grew up in Alaska--since you are writing from Switzerland, my first question is are you gardening in a large heated greenhouse or using some sort of a heated cold frame?
I always assume that your winters in Zurich were probably like my winters in Valdez, very, very snowy. Am I wrong? Where I grew up we usually had snow on the ground before October 31, with an extremely short autumn preceding a very long winter.
Things I am able to grow outdoors in the winter here in Oregon(because we have no snow) are:
brassicas of all kinds, cabbage, kale, brussels sprouts
tat soi and other chinese greens
These plants respond beneficially to a frost, as the frost produces a build up of sugars in the leaves. Makes them very, very yummy!
If you are gardening outdoors I would think it would be important to start these plants at least 6 or maybe 8 weeks prior to planting for any hope of getting a harvest--they need time to mature and harden off before being place outdoors. Additionally, a cold frame or floating row cover would extend your harvest 2-4 weeks.
There are great books about gardening in cold country. It might not be too hard to build a cold frame with heat cords sumberged in the bottom--this is the "new" way to do it, and requires a power source. In the old days people used to take some uncured/unaged manure, dig down a couple feet, and line the pit of the cold frame with the manure, then put a couple feet of soil over the manure, and then plant. The decomposing of the manure provides heat for the cold frame for the winter.
Hope some other true cold-country gardeners respond to your query!