Bake the fries as the label directs. Meanwhile, whisk 1 teaspoon mustard, the vinegar, half the minced shallot and 2 teaspoons vegetable oil in a large bowl; set aside.
Combine the beef, remaining minced shallot, remaining 1 tablespoon mustard, the Worcestershire sauce, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl. Form into four 4-inch patties (about 3/4 inch thick). Heat the remaining 1 tablespoon vegetable oil in a large skillet over medium-high heat. Add the patties and cook until browned, 3 to 4 minutes per side for medium rare; top each with 2 slices brie during the last 2 minutes of cooking and cover to melt.
Toast the buns. Add the watercress to the bowl with the dressing, season lightly with salt and pepper and toss to coat. Put the burgers on the buns and top with the watercress salad. Serve with the fries, cornichons and ketchup.