With a whisk, stir the broth, flour, and mustard in a large nonstick skillet until blended and smooth. Cook over medium-high heat, stirring constantly, until the mixture comes to a boil and thickens, about 2 minutes. Pour the mixture into a small bowl and cover to keep warm. Wipe the skillet clean.

Pat the beef dry with a paper towel; sprinkle with the salt and 1⁄8 teaspoon of the pepper. Heat 1 teaspoon of the oil in the same skillet over medium-high heat. Add half of the beef and cook, turning once, until browned but still pink on the inside, about 2 minutes. With a slotted spoon, transfer the beef to a plate; keep warm. Repeat with the remaining 1 teaspoon oil and the remaining half of the beef.

Add the onion to the skillet. Cook, stirring, until translucent, 3–5 minutes. Add the mushrooms, water, and the remaining 1⁄8 teaspoon pepper. Cook, stirring, until the mushrooms are tender, 3–4 minutes. Add any accumulated meat juices to the sauce mixture in the small bowl. Return the beef to the skillet with the onions and mushrooms. Cook, stirring occasionally, until just heated through, about 1 minute. Transfer the meat mixture to a platter; keep warm.

Re-whisk the sauce mixture and pour it into the skillet. Cook over medium-low heat, stirring occasionally, until the sauce is heated through, about 2 minutes. Remove the skillet from the heat and stir in the sour cream. Pour the sauce over the meat, then sprinkle with the parsley. Serve with the egg noodles.

Serving size: 2⁄3 cup stew and 2⁄3 cup noodles.