To prepare dressing, combine the first 9 ingredients in a blender; process until smooth, scraping sides.
To prepare salad, combine lettuce, cucumber, green bell pepper,red bell pepper, and plum tomatoes in a large bowl; add dressing, tossing gently to coat. Place about 2 1/2 cups salad on each of 4 plates. Sprinkle each serving with 1 tablespoon onion and 1 teaspoon capers.
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