Peel the carrots and celery bulbs and dice them 1.5 cm. Finely chop the leaves from the parsley stalks.
Heat the butter and olive oil in a saucepan. Sauté vegetables in it for 1 minute. Season with salt, pepper and a pinch of sugar. Pour in the vegetable stock, bring to the boil and cook covered for 10 to 12 minutes over a medium heat. Season the vegetables with a splash of lemon juice and mix in the parsley.