Melt butter in a frying pan over medium-low heat. Add onion and garlic. Cook, stirring occasionally, for 10 minutes or until onion starts to caramelise.
Add flour. Cook, stirring, for 2 to 3 minutes. Gradually add stock. Bring to the boil. Reduce heat to medium-low. Simmer for 10 minutes or until slightly thickened. Season with pepper.
