Prep. Transfer the diced radishes, tomatoes, cucumbers, and onions to a salad bowl; set aside.
Make the dressing. In a mixing bowl, combine mayonnaise, yogurt, garlic, lemon juice, salt, pepper, chopped chives, and chopped dill; whisk to combine. If the dressing consistency is too thick, add a tablespoon of water, more lemon juice, or a splash of milk. Taste and adjust accordingly.
Combine and serve. Pour the dressing over the salad and gently toss to combine. If time permits, cool it in the fridge for 15 to 20 minutes before serving.