Heat the butter in a pan, add the flour and sauté a little (should stay light). Deglaze with the cold vegetable stock and bring to the boil while stirring with a whisk.
Add the broccoli and carrots, then let them cook gently for at least 30 minutes.
When the vegetables are soft, add the cream and puree everything very well with the hand blender.
Season to taste with salt and pepper, add a little nutmeg if you like.