Combine vinegar, sugar, and salt in a mason jar or small bowl.
Microwave for 45 to 60 seconds, or until hot. Then stir until sugar and salt are dissolved.
Add red onions, then press down so they're covered in the vinegar mixture. Cover and set aside.
Prepare Dressing:
Combine sour cream, lemon juice, mustard, sugar, garlic, salt, and pepper in a medium bowl. Mix until smooth.
Stir in chives and dill. Then season to taste with additional salt, pepper, and / or lemon juice if needed. Set aside.
Assemble Salad:
Remove stem end from lettuce (if attached), then separate leaves. Gently tear into bite-sized pieces, then wash and dry (use a salad spinner for best results).
Place lettuce, tomatoes, cucumber, radishes, and ¼ cup pickled onions (well drained) in a large bowl. Drizzle with dressing, then gently toss until everything is well mixed and evenly coated in dressing.
Season salad with additional salt and pepper (if needed), then serve immediately. Enjoy!
Notes
I recommend making the onions in advance. They'll taste best if they have at least one hour to pickle. Use the leftover on tacos, sandwiches, other salads, and more.If you have leftover herbs after making the dressing, sprinkle them over the finished salad before serving.Make sure to read the info before this recipe card for helpful tips, recipe variations, serving suggestions, and more!