1. Combine shallot, salt, and vinegar in a bowl and let stand 10 minutes. Whisk in mustard and 3 tbsp. oil and taste. If the vinaigrette is too sharp, whisk in more oil.
2. Toss endive gently with vinaigrette and chives. Arrange leaves in a large bowl or on a platter, sprinkling in blue cheese as you go so it's evenly distributed.
*Find at well-stocked grocery stores.
Make ahead: Vinaigrette, up to 1 day.