How to Make It
Stir together first 6 ingredients in a jar with a tight-fitting lid; let stand 5 minutes. Add buttermilk and next 3 ingredients. Cover jar with lid; shake vigorously until blended and smooth. Add salt, pepper, and sugar to taste.
Toss together radishes, next 5 ingredients, and 1/2 cup dressing in a large bowl; let stand 30 minutes. Season with salt and pepper. Serve with remaining dressing.