This recipe is only really of interest to people living in the Northeastern America, from Maryland through Minnesota, including most of Quebec, Ontario, and Manitoba, or so I understand. Reason being, that's where Garlic Mustard is considered a dangerous invasive weed. It makes zillions of seeds that will mature even on dead plants and is extremely hard to kill. It takes over forest undergrowth and strangles the native species, and it is also biennial, so you have to come back for at least two years after you've attempted to exterminate it to find the next round.
The reason it's here, however, is that it was a potherb, and one way of destroying those seeds is to cook and eat them. So when you've uprooted a bunch of plants (try to get them in the early spring before the white flowers open, since that's when they're the least bitter) give them a good wash:
Then you can hold them by the root and with the crook of the index finger and thumb, strip away the leaves and the flowery crown and/or seedpods:
They're tastier with some onion or garlic already in the soup. In this version, I'm going totally gathered-food, with wild leeks and wild garlic as well. Beware picking wild garlic, though, because white death camas. This is stuff I let somebody else gather for me:
Chop and fry, in butter naturally, the whites of the garlic and leeks:
Add leftover cooked rice as a thickener, and enough water to start cooking the garlic and leeks together. Add salt and/or stock to make as much soup as you want to make, proportional to the greens:
Chop the garlic-mustard leaves and flowers and the green parts of your garlic and leeks, and put half into the soup.
With 1/2 the garlic mustard cooked (about 20 mins light simmer), I like to use a hand-blender to smooth out the base and the rice:
Then finish with the other 1/2 of the garlic mustard and greens to give the soup some texture. Sour cream is a good addition.