How to Make It
Generously season the sirloin with salt and pepper and place into a bowl with the flour. Toss the pieces evenly to coat, lightly tap off the excess flour, and set aside.
In each of 2 large skillets, melt 1 tablespoon of the butter and 1/2 tablespoon of the olive oil over medium heat. Add the mushrooms, onions, and garlic to one pan and the seasoned sirloin to the other. Cook until the mushrooms and onions have lightly browned, about 5 minutes. Cook the sirloin until the pieces are deeply browned, about 3 to 4 minutes per side. Add a 1/4 cup of the white wine to each skillet to deglaze the pans. Transfer the contents from each pan to a slow cooker. Add the beef broth, Worcestershire sauce, and mustard to the pot and cook on high for 3 hours or on low for 6 hours, until the sirloin is very tender.
Bring a pot of salted water to a boil, cook the pasta until al dente and drain. Transfer the pasta to one large serving dish. Stir some butter into the noodles and top with the stroganoff. Swirl in the sour cream and season with salt and black pepper to taste. Garnish with chopped parsley and serve.