Melt butter
in medium saucepan. Add onion and cook gently 3 minutes, stirring. Add
flour and stir to blend. Remove from heat and add milk and chicken stock
all at once. Stir to blend. Return to moderate heat and cook until
boiling, thickened and smooth, stirring constantly. Stir in lemon juice,
mustard, sugar and salt. Keep hot. (If you are making the sauce ahead of
time set it over hot water.) Cook broccoli in a little boiling salted
water until just tender. Serve immediately with mustard sauce spooned
over.
