Warm oil in a large skillet over medium heat. Add fennel, bell pepper and onion. Cook, stirring frequently, until softened and beginning to caramelize, 20 to 25 minutes. Season with salt. Transfer vegetables to a bowl and cover to keep warm.
Place an oven rack about 3 inches from heat source; preheat broiler to high. Line a broiler pan with foil. Broil sausages, turning once, until cooked through, 12 to 15 minutes. Spread mustard on buns, if desired. Place sausages in buns and top with fennel mixture.
