Begin by cutting the beef chuck roast into 1-inch cubes, which allows for even cooking. Set it aside while you prepare to sear it.

In a large skillet, heat a drizzle of oil over medium-high heat. Add the beef cubes in batches, searing each side for about 3-4 minutes until they achieve a deep brown color.

In the same skillet, add the finely chopped onion and minced garlic after removing the browned beef. Sauté for 3-4 minutes until the onions become translucent.

Add the sliced mushrooms and sauté for another 2-3 minutes until they soften and release moisture.

Transfer the seared beef, sautéed onion, garlic, and mushrooms into the slow cooker. Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Season with salt, pepper, thyme, and paprika.

Cover the slow cooker and cook on low for 7-8 hours or on high for 4-5 hours, maintaining a steady cooking temperature.

About 30 minutes before serving, mix sour cream and flour until smooth. Stir this mixture into the slow cooker to thicken the sauce.

Once cooking is complete, taste and adjust the seasoning if necessary. Serve the stroganoff over egg noodles or rice, garnished with parsley.