Juice of 1 orange (preferably a large Spanish Navel orange)
2 tbs lemon juice
2 tbs balsamic or white wine vinegar
1 clove garlic, crushed
Pinch salt
tsp dry mustard powder
Freshly-ground black pepper
1 tsp brown sugar.
Put everything in a screw-top jar and shake thoroughly. Will keep for a couple of days in the refrigerator.