Trim away the outer leaves and coarse stalks from the broccoli. Leave only the tender inner leaves and cut the heads in half if necessary. Add to a pan of boiling salted water and simmer for 15 minutes until tender. Draw off the heat, drain and mash or blend to a purée.
Meanwhile, melt the butter in a large saucepan. Stir in the flour and cook over a low heat for 1 minute. Do not allow