Heat a large skillet over medium heat and melt the 2 tablespoons unsalted butter. Add the 1 large onion and 2 cloves garlic, and sauté until fragrant and translucent. About 6 minutes.
Toss in the 8 ounces cremini or button mushrooms and cook until they have released their moisture and turned golden brown. About 5 minutes.
Sprinkle the 2 tablespoons all-purpose flour over the onion and mushroom mixture, stirring well to coat everything evenly. Cook for a minute or two to cook off the raw flour taste.
Gradually pour in the 1 cup chicken broth and ¼ cup dry white wine, stirring constantly to avoid lumps. Stir in the 1 tablespoon Dijon mustard, 1 tablespoon Worcestershire sauce, 1 teaspoon paprika, 1 teaspoon dried thyme, and 1 teaspoon dried sage. Allow the mixture to come to a gentle simmer, stirring occasionally until it thickens slightly. About 5 minutes.
Reduce the heat to low and add the 2 cups leftover roast pork to the skillet. Stir in the 1 cup sour cream, Let the flavors meld together for a few minutes, but avoid boiling the sauce.
Taste and season with salt and pepper according to your preference.
Serve the leftover roast pork stroganoff over cooked egg noodles, mashed potatoes, or rice, garnishing with fresh parsley for a pop of color and extra freshness.