Step 1

Stir together the olive oil, brown sugar, salt, cumin, paprika, pepper, and garlic, forming a paste; rub the paste into both sides of the steak, using all of the mixture. Place the onion slices in a 5- to 6-quart slow cooker; top with the steak. Pour the beer and soy sauce over the steak. Add the bay leaf and thyme. Cover and cook on LOW until the steak is tender, 7 to 8 hours.

Step 2

Transfer the steak and onions to a platter, reserving the cooking liquid in the slow cooker; cover the steak and onions with aluminum foil to keep warm.

Step 3

Pour the reserved cooking liquid through a wire-mesh strainer into a medium saucepan, discarding the solids. Bring to a boil over high, and boil until the sauce is reduced to about 1 1/2 cups, about 12 minutes. Stir together the cornstarch and water in a small bowl; drizzle into the sauce, and whisk until blended. Reduce the heat to medium-high, and simmer, stirring often, until thickened, about 1 minute. Shred the steak with 2 forks. Divide the steak and onions among the toasted rolls. Pour the sauce into dipping bowls, and serve with the sandwiches.