Place bell pepper on an aluminum foil-lined baking sheet.
Broil 5 inches from heat (with electric oven door partially open) about 5 minutes on each side or until blistered.
Place pepper in a heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skin. Peel pepper; remove and discard seeds. Dice pepper and onion; peel, seed, and dice cucumber. Set vegetables aside.
Whisk oil and next 5 ingredients in a small bowl.
Cook mustard and cumin seeds in a small skillet over medium heat, stirring constantly, 5 minutes or until toasted; immediately stir into oil mixture. Add tomato and cilantro, stirring dressing well.
Bring water and next 3 ingredients to a boil in a saucepan; stir in couscous. Remove from heat; cover and let stand 5 minutes or until liquid is absorbed. Transfer to a serving bowl; stir in diced vegetables. Drizzle with dressing; toss gently.