If needed, cut beef into bite-size pieces and place in a mixing bowl. Season with salt, pepper, garlic powder and paprika. Add flour and toss until evenly coated.
1 1/2 # beef chuck roast, 3 tbsp. flour, 1/2 tsp. garlic powder, 1 1/2 tsp. salt, 1/2 tsp. black pepper, 1/4 tsp. paprika
Heat oil in a large skillet over medium-high heat. Brown beef on all sides, about 4–5 minutes, working in batches if needed. Transfer to the slow cooker.
1 tbsp. olive oil
Melt 1 tablespoon butter in the same skillet. Add mushrooms and onions and sauté for 4–5 minutes, until softened and lightly browned.
1 tbsp. butter, 1/2 cup yellow onion, 8 oz. white mushrooms
Add beef broth, Worcestershire sauce and Dijon mustard. Bring to a simmer, scraping up any browned bits from the bottom of the skillet. Pour everything into the slow cooker.
2 cups beef broth, 1 tbsp. Worcestershire sauce, 1 tsp. Dijon mustard
Cover and cook on LOW for 6–7 hours, or until the beef is fork-tender.
Whisk together cornstarch and cold water until smooth. During the last 20 minutes of cooking, stir the slurry into the slow cooker. Cover, turn heat to HIGH and cook until the gravy has thickened.
2 tbsp. cornstarch, 2 tbsp. cold water
About 5 minutes before serving, place sour cream in a small bowl and stir in a ladleful of hot gravy until smooth. Stir the tempered sour cream mixture into the slow cooker and heat through.
1/2 cup sour cream
Spoon the beef stroganoff over cooked egg noodles and garnish with fresh parsley, if desired.