In a jar, combine white wine vinegar, granulated sugar, and kosher salt. Microwave the mix for about 30 seconds until hot, stirring until the sugar dissolves. Add thinly sliced red onions, cover the jar, and let them pickle at room temperature for at least 30 minutes.

In a medium bowl, scoop in the sour cream, fresh lemon juice, and Dijon mustard. Sprinkle in granulated sugar, minced garlic, kosher salt, and freshly cracked pepper. Whisk these ingredients together until smooth and creamy. Fold in chopped chives and dill.

Remove the core from the butter lettuce and tear the leaves into bite-sized pieces. Rinse the lettuce under cold water, then dry. In a large bowl, combine the lettuce, halved tomatoes, diced cucumber, sliced radishes, and pickled onions.

Drizzle the creamy dressing over the salad ingredients. Gently toss everything together until evenly coated. Taste and season with additional salt and pepper as needed. Serve immediately.