Position a rack in the lower third of the oven and preheat to 500 degrees . In a small skillet, bring 1/4 cup water, 1/4 cup sugar, the vinegar and 1/2 teaspoon salt to a simmer over medium heat. Add the scallions and cook for 1 minute. Let sit for 30 minutes; drain.
Meanwhile, in a small bowl, combine 2 teaspoons EVOO, the garlic and 3/4 teaspoon each salt and pepper. Place the pork tenderloin on a roasting pan and rub with the garlic mixture. Roast for 7 minutes, then turn and roast until it registers 150 degrees on an instant-read thermometer, about 5 minutes. Transfer to a plate and tent with foil. Adjust the heat to broil.
In a small saucepan, heat 1/4 cup water and the remaining 1/4 cup sugar over medium-high heat, stirring, until the sugar dissolves. Add the rhubarb and cook until tender, about 5 minutes; drain.
Thinly slice the pork. Brush the rolls with the remaining 2 tablespoons EVOO, place on a baking sheet and broil until golden. Divide the rhubarb jam among the toasts and top with the pork and cheese. Broil until the cheese melts, about 2 minutes. Top with the quick-pickled scallions and the watercress.