In a jar, combine white wine vinegar, granulated sugar, and kosher salt. Microwave for 30 seconds until warm, then stir until dissolved. Add red onions and let pickle for at least 1 hour.

In a mixing bowl, whisk together sour cream, lemon juice, Dijon mustard, sugar, minced garlic, salt, and pepper until smooth. Stir in chives and dill, adjusting seasoning as needed.

Remove the root end of the butter lettuce, tear the leaves into bite-sized pieces, rinse gently under cold water, and dry thoroughly.

Wash and slice the cucumber, halve the grape tomatoes, and slice the radishes thinly. Combine all chopped veggies in a large bowl.

In the large bowl with chopped vegetables, add dried butter lettuce and well-drained pickled red onions. Drizzle dressing over mixture and toss gently.

Transfer the salad to a serving platter or individual bowls. Season with additional salt and pepper to taste and serve immediately.