Gooey, sweet-sour, silky-soft onion on a crisp puff pastry base topped with salty molten blue cheese, is a superb combination. Make these lovely tarts as a starter or light supper with salad. Highly recommended. Serves 4 Prep: 30 min Cook: 40 min 3 large onions 3 tbsp olive oil 1 tsp dark brown sugar ½…