30 mL (2 tbsp) scallions, green part only, sliced thinly
Salt and freshly ground pepper, to taste
Snap off the tough ends of the asparagus and discard. Cut off the tips, leaving 2.5 cm (1 in) of the stalk. Cut the rest of the stalks in pieces. Keep tips and stalks separate.
In a saucepan, sweat the leek in the butter for 10 minutes over low heat. Add the potato and stock. Season to taste, bring to a boil, then reduce heat and simmer 10 minutes.
Add the asparagus stalks and continue cooking 8 minutes. Add the tips, reserving 8 for the garnish. Continue cooking 5 minutes more.
Pour the hot mixture carefully into the blender and pulse to purée. Pour into a saucepan and set aside. Blanch the 8 remaining asparagus tips and set aside.
While the soup is cooking, prepare the flans.
Preheat oven to 180°C / 350°F. Butter 4 small ramekins, cover the bottoms with parchment paper and butter the paper.
In a bowl, whisk together the flan ingredients and pour into the ramekins. Place the ramekins in a baking dish and pour very hot water into the dish, halfway up the sides of the ramekins. Cover the flans with parchment paper. Bake about 55 minutes or until a knife inserted in the middle comes out clean. Allow to set on the counter in the bain-marie until the water is lukewarm. Unmold the flans and place on a piece of parchment paper.
Serve the soup piping hot in deep soup plates, garnishing each serving with one room-temperature flan and two asparagus tips.