I've always loved any kind of stroganoff -- it's one of my comfort foods. This is my go-to recipe. I generally make it with ground beef, because it is a little more forgiving in the cooking: leftovers tend to be a little dry when you use stew meat or stir-fry beef. You can also substitute chicken and chicken broth, or go vegetarian and use just mushrooms and vegetable broth!
In a large skillet, over medium heat, sautee onion in oil until translucent, approximately 5-6 minutes.
Add meat and sautee until browned.
Add mushrooms and cook 5-6 minutes, until mushrooms are softening and releasing their water.
Add flour and stir well to combine completely. Let simmer for 2 minutes.
Add mustard and broth, stirring well to combine. Start with 1/2 cup of broth, adding more until consistency of gravy is to your liking. Keep in mind it will thicken as it cooks. Reduce heat to low and simmer for 20 minutes, stirring occasionally.
Turn off heat and add sour cream, stirring well to combine. Serve over rice or buttered parsley noodles.