Clean broccoli, wash it, cut it into florets. Peel the stalk and dice finely. Melt butter in a pot. Sauté broccoli in it. Add broth, bring to the boil and simmer for about 8 minutes.
Take out 2 florets and put them aside. Heat the oil in a pan and fry the tortellini for about 3 minutes. Add 100 ml water and simmer for 1 minute. Mix buttermilk with starch.
Puree the soup and bring to the boil again. Add buttermilk while stirring. Season to taste with salt and pepper. Divide broccoli florets into smaller florets. Pour tortellini into the soup, add broccoli florets and arrange in bowls.
Garnish with chervil.