Vietnamese Steak Salad

To prepare steak: Combine lime juice, fish sauce, soy sauce and chile oil in a large sealable plastic bag. Add steak and seal the bag, turning to coat the meat with the marinade. Marinate in the refrigerator, turning the bag occasionally, for 4 hours. Meanwhile, prepare vinaigrette: Combine broth, lemongrass and 2 tablespoons shallot in a small saucepan. Cook over medium heat until reduced to 1/4 cup, about 10 minutes. Remove from the heat and let stand for 10 minutes. Strain into a small bowl, discarding solids. Add the remaining 2 tablespoons shallot, lime juice, fish sauce, soy sauce, sugar and chile oil; whisk to combine. Just before serving, stir in cilantro and mint. Preheat grill to high. Remove the steak from the marinade (discard marinade). Grill the steak, 4 to 5 minutes per side for medium-rare. Transfer to a clean cutting board and let rest for 5 minutes. Slice thinly on the diagonal, across the grain. To prepare salad: Toss watercress (or spinach), cucumber, carrot, cilantro and mint with 1/2 cup of the vinaigrette in a large shallow bowl. Arrange the steak over the greens. Spoon the remaining vinaigrette over the top and serve.