To Make Dressing:
Whisk together vinegar, shallot, salt, mustard, and pepper in a small bowl. Add oil in a slow stream, whisking constantly until dressing is emulsified. Whisk in chives.
To Make Salad:
Toss together endive, mache, and radishes in a large bowl. Drizzle with dressing and gently toss to coat. Adjust seasoning if desired. Serve immediately.
* Mache may be found at specialty markets and food stores. Bibb or Boston lettuce or argula, washed, dried and cut may be substituted.