Pat beef cubes dry and sprinkle with salt and pepper. Heat oil in frying pan large enough to sear in one layer (or work in batches).
Add beef and brown over medium high heat. About 4 minutes without stirring. Then flip the cubes and cook til browned.
Transfer beef to the slow cooker. Melt butter in the skillet, add onions and cook about 3 minutes. Add garlic and cook one more minute.
Sprinkle flour over the onions and garlic and stir to coat. Add the beef stock.
Transfer all to the slow cooker and cook on low 8 hours or high 5 hours.
Melt butter in large skillet. Add the mushrooms, garlic, salt and pepper. Cook over medium/high heat until liquid is released and mostly evaporated. Set aside until the last 40 minutes of beef cooking.
Add mushrooms, sour cream, Dijon mustard and paprika during last 40 minutes of cooking. Stir to mix well.
Serve over broad egg noodles or rice.
Variation: To cook in the oven transfer the beef mixture to a Dutch oven and cook covered at 350° for 2 hours. Add mushrooms, sour cream and paprika during last 30 minutes of cooking. Stir to mix well.
Variation: to cook on stovetop add beef mixture to Dutch oven. Cover and simmer on low/medium low for 1 ½ to 2 hours. Stir occasionally. When beef is fall apart tender it is ready.