Beef Stroganoff

Prepare the Beef - Slice the steak against the grain into about ¼-inch thick and 2-inch long strips.
Cook the Beef - Heat a large skillet over medium-high heat. Add the olive oil. Once hot, add the beef slices in a single layer. Sear until lightly golden brown, 1 to 2 minutes. Saute until medium rare, for about 1 to 2 minutes. Season with ½ teaspoon salt and ¼ teaspoon black pepper. Transfer to a clean plate.
Cook the Mushrooms - Melt the butter over medium heat. Add the garlic, and saute until fragrant, 30 seconds. Add the onions, and saute until tender, about 2 minutes. Add the mushrooms and saute until tender, about 2 minutes.
Make the Sauce - Add the flour, stir, and cook for 1 minute. Slowly stir in the beef broth. Bring to a rapid simmer over medium-high heat. Stir and cook until the sauce is thickened, about 2 minutes.
Reduce the heat to low and stir in mustard, sour cream, ½ teaspoon salt, and ¼ teaspoon pepper. Taste and season with more salt and pepper as desired.
Warm the Beef - Add the cooked beef to the sauce, and heat until warmed, 1 to 2 minutes.
To Serve - Garnish with parsley. If desired, serve with egg noodles, mashed potatoes, or rice.