In a 10-inch skillet, add the asparagus; add enough water to cover. Bring to a full boil. Cook over medium-heat until asparagus are crispy tender (3 to 5 min.) Drain; return to skillet.
Add the remaining ingredients to the skillet, pushing the asparagus to the side just until the butter has melted. Cook over medium heat, stirring occasionally, until heated through and the mushrooms have softened (3 to 4 minutes).
Notes
If you are preparing this recipe as gluten-free, be sure to use a brand of mustard that is known to be GF.