Trim the beef and cut into 1.5 inch / 3.5 cm cubes.
In a large bowl whisk the flour with the salt, pepper and nutmeg. Toss the beef cubes in the flour.
Add 1 tablespoon of oil to a large frying pan / sauté pan over a high heat sauté the beef cubes until brown all over – this takes around 4-5 minutes. Remove from the pan with a slotted spoon and place in the slow cooker.
Turn the heat down then add the other 1 tablespoon oil to the sauté pan / frying pan and sauté the onion and mushrooms for 5 minutes.
Add the minced garlic and cook for 1 minute.
Take the pan off the heat and stir in the Dijon mustard, thyme and beef stock. Pour this mixture over the beef in the slow cooker.
Cook on low for 8 hours, until the beef is tender.
Once the beef is tender, add the sour cream: in a small bowl mix the sour cream with 3 tablespoons of the hot stroganoff sauce. Gently stir this back in the stroganoff, taking care as the beef will be delicate.
Check for seasoning.
Serve over rice, noodles, pasta or mashed potato potatoes, sprinkled with freshly chopped parsley or chives.