Bring 6 cups water up to a boil and salt generously. Rinse basmati rice in a fine strainer and add to boiling water.
Cook uncovered, for exactly 10 minutes, then strain and rinse under cold water until cooled.
Whisk mayonnaise, sour cream, mustard and white wine vinegar together. Stir in chopped green onion and herbs and season to taste.
Toss cooled rice, peas and radishes together and dress. Adjust seasoning if necessary and chill until ready to serve.
Dressing can be prepared a day in advance and salad can be tossed up to 4 hours ahead of serving.