Finely dice the onion. Peel the carrots and celery and cut into fine sticks. Wash the leek and cut into thin rings.
Fry the onion in a saucepan with the butter. Add the vegetables and cook for about 3 minutes. Pour in the broth and cook for 15 minutes in an open saucepan over a mild heat.
Season the soup with salt and pepper. Pour in the tortellini and let simmer for 5 - 6 minutes over low heat.
Since my daughter has been a vegetarian, I`ve been swapping meat broth for vegetable broth and using tortellini with a cheese filling. Tastes very good too!