In a food processor puree the trout, the yogurt, the mayonnaise, the lemon juice, the minced dill, the onion, and salt and pepper to taste until the mixture is smooth. The trout mixture may be made 1 day in advance and kept covered and chilled. On each piece of bread spread a scant teaspoon of the mixture, mounding it, and garnish each canapes with a dill sprig. Makes about 36 canapes. Gourmet September 1991