Scoop out meat from avocados and mash it in a bowl with a fork. Pour in lime juice and combine well with a spatula. Incorporate the cream cheese, mixing it thoroughly with the avocados. Add the watercress, scallions, water chestnuts, pistachios, cayenne, black pepper, Worcestershire sauce and salt. Mix well.

Measure 2/3 cup cold water in a cup. Stir in the gelatin, mix and let it rehydrate for a minute or two. Add 1/3 cup boiling water and stir until it dissolves. Pour gelatin into the avocado mix, incorporating it with a spatula.

Lightly grease a ring mold with olive oil. Pour the avocado mix into the mold. Shake the mold softly a couple of times to level the mix. Cover it well and place it in the refrigerator until it is set, for at least 3 hours. You can leave it overnight or until you are ready to unmold. The avocado mousse will last beautifully in the refrigerator for 2 days. When ready to unmold, remove from the refrigerator, run the tip of a knife along the edges and flip onto a plate. You may need to shake the mold a couple of times, holding onto the plate as you do so.

You may serve it on a platter, retro style, with some watercress leaves in the center of the ring or on top. Or serve it already sliced with a side of smoked salmon and pieces of toast.