Pop the beef cubes into a bowl with the flour and a pinch of salt and pepper. Toss it all to coat the beef in the flour.
Heat a cast iron skillet over a medium to high heat and drizzle in the olive oil. Add in the beef and brown for 5 minutes or until it is brown all over.
Take the beef out of the skillet then add in the butter and let it melt. Pour in the onion, garlic and mushrooms and sauté for 5 – 8 minutes. Then pour in the white wine and deglaze the pan.
Pour the mushroom mix into the slow cooker along with the browned beef, beef stock, mustard, and Worcestershire sauce. Stir it all together and replace the lid. Cook on high for 4 – 5 hours or on low for 8 – 10 hours.
Mix the cornstarch and water together and once the meat is soft, pour the cornstarch in and mix it into the stroganoff. Then pour in the sour cream and creamy mushroom soup and mix it in.
Turn the slow cooker on high and cook for another 30 minutes until it is thick. Then stir in the cooked egg noodles.