Prepare the vegetables: Wash the radishes and cucumber thoroughly, then slice them thinly. A mandoline works best for even slices. Place them in a large mixing bowl along with the chopped spring onions.
Make the dressing: In a small bowl, whisk together sour cream, olive oil, Dijon mustard, honey, lemon juice, parsley, and seasoning until smooth and well combined. Adjust with chili flakes for a touch of heat if desired.
Assemble the salad: Pour the dressing over the sliced vegetables and toss gently to coat everything evenly.
Finish and serve: Taste and adjust the seasoning. Add more lemon juice if you prefer a brighter, zesty flavor. Serve immediately for the freshest taste.