We love this recipe. It's fast, easy, tastes good, and is healthy. I found it on one of my favorite blogs, CuisineNie (aka nieniedialogues.blogspot.com), but, since CuisineNie is now closed, I thought I'd put this recipe back out there. The amounts of onion, mushrooms, garlic, spinach, vinegar, and cheese are all approximate and should be altered according to your tastes. The recipe is pretty flexible; you can use different types of mustard, cheese, etc.. We like this served with canned Progresso lentil soup.