In a bowl, combine sour cream, mayonnaise, salt, pepper, and sugar.

Stir the mixture well until smooth and well combined.

Add the broccoli flowerets, cauliflowerets, chopped tomato, and chopped green onions to the bowl with the mayonnaise mixture.

Gently fold all the ingredients together, ensuring the vegetables are evenly coated with the dressing.

Cover the bowl and chill in the refrigerator for at least one hour, or preferably overnight, to allow the flavors to meld.

Before serving, line a serving bowl with fresh lettuce leaves.

Spoon the chilled salad into the lettuce-lined bowl.

Serve immediately and enjoy!