Watercress Soup with Parmesan Flan

Name: Watercress Soup with Parmesan Flan
Yield: For 6
Cuisine: Irish
Ingredients: salt and white pepper; 1 cup (250 ml) dry vermouth; 3 cups (750 ml) (Home Made) vegetable stock; 3 spring onions or large scallions, finely chopped; 2 tbsp olive oil; ⅓ cup (75 ml) heavy cream, if you wish; 2 bunches watercress, washed and stemmed; 2 (3-inch/7.5-cm) potatoes, peeled and diced; 2 large egg yolks; salt and freshly ground black pepper; 1 large egg; ¾ cup (75 g) grated Parmesan cheese, plus some shaved Parmesan for garnish; ½ cup (125 ml) whole milk; ¾ cup plus 1 tbsp (200 ml) heavy cream