COMBINE vegetables; season with salt and pepper. Cover to keep warm.
FOR each serving: Grill baguette, cut-sides down, until golden brown. Spread cut sides of baguette with 2 Tbsp. of the dressing. Toss 1/2 cup of the lettuce with 2 tsp. of the remaining dressing.
PLACE 6-1/2 oz. of the vegetable mixture on bottom half of baguette; top with the lettuce mixture and 1-1/2 oz. each of the cheese and prosciutto. Cover with top of baguette.