1. Heat 1 tablespoon of the oil in a 10-inch skillet. Add the onions, mushrooms, and garlic. Cook over medium heat, stirring occasionally, until the onions are tender. Remove from the skillet.
2. Combine ¼ cup of the flour, salt, and pepper in a large bowl. Add the beef strips and toss to coat. Transfer the beef to a plate, shaking off any excess flour.
3. Return the skillet to the stove over medium-high and add the remaining oil.
4. When the oil is hot, add the beef to the pan in a single layer. Cook for 2 minutes, until golden brown. Flip the beef and sprinkle it with the remaining tablespoon of flour.
5. Add the beef stock, mustard, Worcestershire sauce, paprika, and peppercorns. Simmer 10 minutes, until the gravy is thickened.
6. Return the mushrooms and onions to the pan and remove from heat.
7. Add the sour cream to a small dish. Stir in a few tablespoons of the gravy to warm it up (this will prevent it from curdling.) Stir the warmed sour cream into the Stroganoff, stirring constantly.
8. Taste and adjust the seasoning as needed. If the flavors seem flat, a generous pinch of salt will help bring it together and perk everything up.

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