In a medium bowl , mix Brussels sprouts, tomatoes, pearl onions, French dressing, salt and pepper. Cover and let stand at room temperature approximately 2 hours, stirring occasionally.
Preheat an outdoor grill for high heat.
Transfer Brussels sprouts mixture to a heavy medium skillet. Cover and cook on prepared grill 10 minutes. Uncover and continue cooking 5 minutes, stirring occasionally, until Brussels sprouts are tender.