Remove the tough stems from 2 bunches watercress; wash and dry well, then transfer to a bowl. Heat 3 tablespoons peanut oil in a small skillet over high heat until very hot. Add 1 tablespoon chopped peeled ginger, 1 minced garlic clove and 2 teaspoons Sriracha and stir-fry 30 seconds. Carefully stir in 2 tablespoons rice vinegar. Pour the warm dressing over the watercress and toss. Season with salt.