Whisk together the mustard, lemon juice, olive oil, salt, and pepper to make a vinaigrette. Remove a 1/2-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.
Note: Haas avocados are the brown ones from California. The green ones don't have nearly as much flavor.