Combine the onions and butter over low heat in a medium saucepan. Cook gently, stirring occasionally, till softened. Once softened add a good pinch of sage.
Sift in the flour and stir well. Cook for a few minutes longer, stirring.
Combine the beef stock cube with 600ml of boiling water and add to the pan, whisking well to prevent lumps. Increase heat to medium and bring to a simmer, then reduce heat and gently simmer for 20 minutes, or till desired consistency is reached.