Sardines make a wonderful (and affordable) flavor base for creating a fish "pate" that can be spread on toasts or crackers at a party. Here the sardines are first mashed with a fork and then mixed with chopped onion, plain yogurt, a tiny bit of lemon juice and mustard, and salt to taste to form a creamy spread. Chill until ready to serve; this is a great make-ahead option for entertaining.