With the rack in the middle position, preheat the oven’s broiler.
On a baking sheet, toss the peppers, onion and oil. Roast for about 15 minutes or until the vegetables are al dente. Add the vinegar and mix well. Continue cooking for about 5 minutes. Set aside.
In a bowl, combine the duck, mayonnaise and mustard. Spread the inside of the baguette with the butter. Garnish with the tomato. Cover with the duck and pepper mixtures. Add the tarragon. Close the sandwich. Cut the sandwich into four equal portions.