1. Combine peppers, onion, sausages, vinegar, thyme and black pepper in a 5- to 6-quart slow cooker.
2. Cook on HIGH for 3 hours or LOW for 6 hours. Use a slotted spoon to remove peppers and sausages from slow cooker; set aside and keep warm.
3. Heat broiler. Pour liquid from slow cooker into a small saucepan. Whisk in cornstarch mixture and mustard. Bring to a boil; boil for 1 minute or until thickened.
4. Meanwhile, slice rolls almost all the way through lengthwise. Place on a baking sheet, cut side up, on top rack under broiler for 1 to 2 minutes or until toasted. Cut sausages diagonally into 1/2-inch-thick slices and stir back into pepper mixture. Place a scant 1 cup of the sausage mixture on each roll. Drizzle each sandwich with a generous 2 tbsp of the sauce, reserving remaining sauce for dipping. Serve immediately.