Heat 1 tablespoon Vegetable oil in a pan over a medium heat. Cook 700 g Beef until browned all over. Remove form the pan and set aside.
Melt 25 g Butter in the pan and cook 150 g Mushrooms and 4 Shallots until softened, about 5 minutes.
Add the beef, 2 Garlic clove and the cooked mushrooms and shallots to a slow cooker pot.
Add 2 tablespoon Worcestershire sauce, 2 tablespoon Wholegrain mustard and season with 1 Pinch Sea salt and ground black pepper. Pour in 350 ml Beef stock to cover the meat. Cook on high for 5 hours.
When cooked, add 1 tablespoon Cornflour (corn starch) and 250 ml Sour cream to thicken the sauce.
Serve with 1 tablespoon Paprika and Fresh parsley sprinkled on top (optional).
Notes
You can make this lighter by stirring in Greek yogurt instead of sour cream, however this a risk of it splitting.
Make sure you add the sour cream at the end, to avoid it splitting.
You can swap wholegrain mustard for Dijon if that's all you have.
You could swap some of the stock for white wine if you like.
You can make this with ground beef instead. Fry it in a pan with a little oil and then drain off any excess and add to the slow cooker at the same time you would the diced beef.
We added parsley at the end, however you can swap that for chives.
Rather than serving it on mash or rice, you could add chunks of potato to the slow cooker and cook it all together.
If you sauce is too thin, add a little more cornflour. You don't want to add too much though, as it will taste floury.
Don't stir it too aggressively when you add the sour cream, as the beef might fall apart too much.