Whisk together the mustard, lemon juice, olive oil, salt, and pepper to make a vinaigrette.
Remove a half-inch from the stem end of each endive, discard the core, and cut the rest across into 1-inch chunks. Cut the avocados into large dice or wedges.
Toss the avocados and endive with the vinaigrette. Season to taste and serve at room temperature.
Giangi's Kitchen provides nutritional information, but these figures should be considered estimates, as a registered dietician does not calculate them.