Garlic Mustard is an invasive wild edible so harvest to your hearts content! The tender stalks make a delicious pesto and the leaves can be added to soups. Try to harvest away from roadsides to minimize contaminated runoff.
I did not harvest this plant, but the friends that did were told what they had by a native to these mountains. That person claimed that branch lettuce was becoming rare due to over harvesting. It can be eaten raw in a salad, or cooked in bacon fat.