1. Slice beef into 2-inch-long strips, each about 1 inch wide and 1/4 inch thick. Season generously with salt and pepper. Heat 2 tablespoons oil in a large skillet over high heat. Brown meat in two batches, about 1 minute per side; remove from skillet, and set aside.
2. In same skillet, heat remaining tablespoon oil over medium heat. Add onion; cook, stirring occasionally, until lightly browned, 5 to 7 minutes. Add mushrooms; cover. Cook until just tender, 6 to 8 minutes. Add broth; simmer, uncovered, until liquid has thickened, 6 to 8 minutes.
3. Stir mustard into mushroom mixture in skillet. Return beef to skillet; cook over medium-low heat until heated through, 1 to 2 minutes. Remove from heat, and stir in sour cream. Season with salt and pepper, as desired. Serve immediately, garnished with chopped dill.

Serves 4.