Cut the chuck roast into large chunks. Heat a skillet over medium-high heat and sear the beef on one or two sides until browned. The inside should still be raw. Transfer the beef to the slow cooker.
Add the sliced onion and minced garlic to the same skillet. Cook while scraping up the browned bits left behind by the beef. Add a small splash of water or a little oil if the pan is dry, then transfer the onion and garlic to the slow cooker.
Whisk together 4 cups of water, the au jus mix, and brown gravy mix until smooth. Pour the mixture over the beef and onions.
Cover and cook on high for 6 hours or low for 8 hours, until the chuck roast is tender enough to shred.
Shred the beef directly in the slow cooker. Keep the slow cooker set to high or low, not warm, so the liquid remains hot enough to cook the noodles.
Add the dried egg noodles and stir. Cover for 15 minutes, then stir again. Continue cooking and stirring every 15 minutes until the noodles are tender and have absorbed nearly all the liquid.
Stir in the heavy cream once the noodles are fully cooked. Serve warm.