Mugwort (Artemisia vulgaris) is one of about 300 species in the Artemisia genus. It's what you want for this soup; the others may be too bitter. Watercress will also work; although the soup won't taste the same, it will still be delicious.
Excerpted from "Foraged Flavor: Finding Fabulous Ingredients in Your Backyard or Farmer's Market, with 88 Recipes" by Tama Matsuoka Wong, Eddy Leroux and Daniel Boulud.
"My mom always made a casserole of pork chops layered with stuffing and cream of mushroom soup. I use breadcrumbs to crust the chops, and instead of hiding the mushrooms in a sauce, I roast them until golden and toss them with arugula." -- Lesley Stockton, Food Editor
These mushroom cups can be frozen for up to one month and popped directly into the oven from the freezer (add two minutes to the cooking time). If you put them in while the meat is resting, they'll be ready just in time for dinner.