In a medium bowl, combine the sour cream, flour, water, bouillon granules, pepper, and dressing mix. Set aside.
In a large skillet over medium-high heat, melt butter. Cook the sirloin until almost browned, stirring occasionally. Add onions and mushrooms and continue cooking until vegetables are tender and mushrooms are browned.
Stir in sour cream mixture and cook, stirring frequently, until bubbly and thickened. Serve over noodles and sprinkle with parsley, if desired.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/best-beef-stroganoff/