Heat 1 tablespoon of olive oil in a frying pan over medium heat. Add the sliced onions and grated garlic, gently frying for 4-5 minutes until they're softened and fragrant.
Increase the heat and add the sliced mushrooms to the pan. Cook for 2-3 minutes until the mushrooms are tender and have released their juices.
Add your chosen beef stock, tomato puree, smoked paprika, dijon mustard, and dried herbs to the pan. Stir all the ingredients well to combine and bring to a simmer.
Reduce the heat and allow the mixture to simmer for about 10 minutes, letting the flavors meld together.
Stir in 200ml of creme fraiche (or soured cream) until well combined. Then add the leftover roast beef, cut into strips, and stir everything together. Simmer for an additional 5 minutes until heated through.
Before serving, sprinkle chopped parsley on top for garnish. Serve your Beef Stroganoff with rice, bread, or mashed potatoes, and accompany it with a green vegetable like kale or broccoli.