Boil beans in salted water for 5 minutes, or until crisp-tender.
Drain and refresh in a bowl of ice and cold water. Drain and pat dry. (may be prepared up to 1 day in advance).
In a blender, puree capers with lemon juice, mustard and 2 tblsps white wine. With motor running, add oil in a thin, steady stream and blend until emulsified. Taste and adjust seasoning by adding salt and pepper. (may be made up to 1 day in advance. Whisk before using).
In a large serving bowl, toss together beans and dressing. Sieve hard-boiled egg over top.