Preheat oven to 325 degrees F. Brush 1 tablespoon olive oil on the tomatoes (reserving the rest) and roast for about 25 minutes. Remove from oven, allow to cool, and cut into wedges.
Boil couscous until al dente as you would for pasta, drain well, and toss with 1 tablespoon reserved olive oil.
For the dressing, add the vinegar to a blender and replace the cover. Turn on the blender and through the feed opening add, 1 at a time, the garlic, mustard, salt, and pepper. Leaving the blender running, add the olive oil in a slow thin stream. Set aside briefly.
Toss together the couscous, tomatoes, bell pepper, mint leaves, cipollini onions, and cucumber with enough of the dressing to coat. Serve additional dressing on the side.