Haricots verts pair well with lamb; their crunch and acidity balance the richness of the meat.
Bring a large saucepan of water to a boil. Prepare an ice-water bath. Add salt to boiling water, then add haricots verts; cook just until beans are bright green all over, 1 to 2 minutes.
Drain beans; rinse immediately with cold water. Transfer to ice-water bath. When chilled completely, drain; pat dry.
Put shallot and vinegar in a small bowl; season with salt and pepper. Let stand 15 minutes. Whisk in mustard. Whisking constantly, pour in oil in a slow, steady stream until emulsified. Set aside until ready to dress beans; whisk before using.
Blanched haricots verts can be refrigerated, in a resealable plastic bag, up to one day. Refrigerate the vinaigrette separately, up to one day; bring to room temperature before serving.