Melt butter in medium saucepan. Add onion and cook gently, stirring 3 minutes. Add flour and stir to blend. Remove from heat and stir in milk and chicken stock. Stir to
blend. Return to moderate heat and cook, stirring, until boiling, thickened and smooth. Stir in lemon juice, mustard, sugar and salt. Keep hot.
Cook broccoli in a little boiling salted water until just tender. Serve immediately with sauce spooned over top.