Combine roasted red peppers, walnuts, vinegar, paprika and garlic powder in a food processor; process until a slightly chunky spread forms, 2 to 3 minutes, stopping to scrape down sides as needed and adding liquid from the pepper jar, 1 teaspoon at a time, as needed to help puree the mixture.
Spread the pepper mixture evenly on cut sides of ciabatta rolls (about 2 tablespoons each). Top each bottom slice with ½ cup arugula, 2 tablespoons sliced onion, 2 mozzarella slices and 2 tomato slices. Sprinkle with salt. Top with the remaining ciabatta slices. Cut the sandwiches in half and serve. Additional reporting by Carrie Myers, M.S. and Linda Frahm
