A delicious and pretty salad green that has gone through winter in Salt Lake City (zone 6) for me is Mizuna. It's a type of mustard green but not spicy, and this year I got seeds from Baker Creek of two different colors, chartreuse green, and dark red. Planted them in my daughter's garden in Utah and will see how they're doing next month when I visit again.
One winter it even survived under snow for me, and came up smiling. You can harvest leaves individually, to keep the heads going and looking nice, if you have a row of it. If you only have one or two, you might use it up before winter is over.
Not sure if this qualifies as "evergreen" because I doubt it would last past about 2 years, but anything green you can harvest from your garden in the middle of winter sure is a treasure.
