Prepare the Beef: Cut the chuck steak into 2-inch cubes.
Pat the cubes dry with a paper towel for a perfect sear.
Sear for Flavor: Heat the olive oil in a large frying pan over high heat.
Add the steak cubes and brown on all sides, about 5 minutes.
Don't overcrowd the pan; work in batches if necessary.
Load the Slow Cooker: Transfer the seared beef to the slow cooker.
Add the peeled radishes, minced garlic, diced onion, diced tomatoes, heavy cream, chicken stock, and ground mustard.
Set It and Forget It: Stir all the together.
Cover with the lid and cook on HIGH for 8 hours, until the beef is incredibly tender.
Garnish and Serve: Once cooked, give the stew a good stir. Ladle into bowls and garnish with fresh chopped parsley before serving. Leftovers can be stored in the refrigerator for up to 3 days.
Don't skip the sear! Browning the beef before adding it to the slow cooker creates a deep, caramelized flavor base that you can't get from just slow cooking.