In a small bowl, whisk olive oil, vinegar and mustard. Stir in shallots, 1/4 tsp of the salt and the pepper.
Bring a pot of lightly salted water to a boil. Add asparagus; cook for 1 minute. Drain and run under cold water to cool.
In a large bowl, toss asparagus with watercress, arugula and radishes. Right before serving, stir in remaining 1/4 tsp salt and the dressing.