In a bowl, combine the mayonnaise, mustard, lemon juice, garlic, hot sauce, and salt. Whisk to combine. Fold in the green onions, tomatoes, capers, shallots, and parsley. Gently fold in the crabmeat, turning to gently coat with the dressing and being careful not to break up the lumps. Adjust seasoning, to taste.
Spoon a heaping tablespoon of the stuffing into endive and radicchio leaves. Arrange the watercress and stuffed endives and radicchio decoratively on a platter and serve as an hors d'oeuvre. Alternately, place the watercress on 8 small plates and drizzle with extra-virgin olive oil.
Yield: about 40 hors d'oeuvres