Peel and finely chop the onion. Clean and wash the broccoli and divide into small florets. Peel and finely dice the stems.
Heat the fat in a saucepan. Sauté onion in it until transparent. Steam the whole broccoli briefly. Season with salt and pepper. Add 1/2 l water and milk and stir in the broth. Bring to the boil, cover and simmer for 5-6 minutes.
Cook the tortellini in boiling salted water for about 2 minutes. Drain and let drain well.
Remove about 2/3 of the broccoli florets with a skimmer. Finely puree the rest in the soup with a hand blender. Stir in crème fraiche. Season soup with salt, pepper and nutmeg. Divide the remaining broccoli into even smaller florets.
Heat the tortellini in the soup. Garnish with chervil.