This dressing is inspired by Italian tonnato, a sauce made from pureed tuna, customarily served over thinly sliced veal. Imported Genova tonno (tuna) works well in this recipe.
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.