Season the chuck with salt and pepper and brown it in oil in a hot skillet for 4 to 5 minutes per side.
Put the onions and garlic in a 6-quart slow cooker and set the roast over them.
Whisk the broth with 3 tablespoons horseradish, Dijon, Worcestershire and thyme. Pour it around the roast.
Cover and cook on low for 8 hours until fork-tender. Move the beef to a platter and skim the onion liquid.
Combine cornstarch and water, stir into the cooker and cook on high for 10 minutes. Stir in the remaining 1 tablespoon horseradish, then serve the gravy over sliced or pulled beef.