In a food processor, pulse red bell pepper, scallions, cilantro, garlic, coriander, cumin, harissa, and salt until vegetables are very finely chopped. Add beans; pulse until beans are coarsely mashed.
Transfer mixture to a bowl; add breadcrumbs and egg. Mix with your hands until well combined. Shape into 8 (¼-cup) burgers. Place on a waxed-paper-lined plate and refrigerate 30 minutes.
Heat oil in a large skillet over medium heat. Add sliders and cook 3 to 4 minutes per side, or until golden brown. Drain on a paper towel.
Cut 1 inch off the tops of pitas. Spread insides with harissa mayonnaise, then line each with a lettuce leaf, a tomato slice, and a slider.
