Peel outer leaves from lettuce. Remove core. Rinse thoroughly and drain. Cut lettuce into 6 to 8 wedges. Place on a serving platter; cover and chill up to 4 hours.
For dressing, in a medium bowl combine cucumber, onion, sour cream, mint, vinegar, the 1 teaspoon dill seed, salt, sugar, if desired, and pepper. Stir to combine. Cover and chill 1 to 4 hours to blend flavors.
To serve, spoon dressing over each wedge. Garnish with a cherry tomato. Sprinkle with additional dill seed, if desired. Makes 6 to 8 servings.