Step 1

Pulse the scallions, thyme, garlic, 1 teaspoon each salt and pepper, the celery seeds and allspice in a food processor to make a paste. Pierce the roast all over with a paring knife; rub the spice paste into the meat. Tie the roast with kitchen twine so it keeps its shape.

Step 2

Spread the carrots in a 6-quart slow cooker and set the roast on top. Insert an instant-read thermometer at an angle into the center of the roast; position the thermometer so it can be easily read through the lid of the slow cooker.

Step 3

Combine 1 1/2 cups water, the Worcestershire sauce and bouillon cube in a microwave-safe bowl; microwave until hot, 2 to 4 minutes. Add to the slow cooker. Cover and cook on low until the meat registers 120 to 130 degrees F for medium rare, about 2 hours. Transfer the meat to a cutting board and cover with foil; let rest 15 minutes. Skim off the excess fat from the cooking liquid.

Step 4

Remove the twine and slice the meat very thinly. Spoon some cooking liquid onto the cut sides of each roll; sandwich with the meat, horseradish and/or mustard. Serve with more cooking liquid for dipping.