As you cut avocados drizzle lemon juice on them so that they will not darken. Mash avocados in a bowl and add the mustard and worcestershire sauce.
In a separate bowl lightly whip the 1/2 pint of whipping cream. The cream should be a little loose, soft peak stage is the technical term. Fold cream into avocado mixture.
Place parchment paper in a standard bread pan so that there is enough paper to cover the top of the filled terrine. Spray the parchment with release spray. Fill the terrine with 1/2 of the mixture, and refrigerate for 20 to 30 minutes.
Remove terrine from refrigerator, slice the extra avocado and drizzle lemon juice on it to keep it from darkening, place the slices on the top of the chilled mixture. Pour other 1/2 of the avocado melange on top. Drizzle some lemon juice on top and fold parchment paper over top.
Refrigerate for 35 to 40 minutes. While the terrine is chilling, peel devein and barely poach the shrimp, let the shrimp cool in the poaching liquid if possible. Set the shrimp aside for the garnish. Separate nice looking basil leaves for garnish.
Prepare the sauce; roughly cut the tomato into bite size pieces, place in a small frying pan or saucepan and stew them slightly. Add a little lemon juice and the cream, mix in a few stray leaves of the basil cut into strips. The goal is to quickly cook off some of the liquid. Let this sauce cool to room temperature or below before saucing the dishes.
To unmold, sit the terrine in some warm water for 1 minute or less, and unmold onto a platter. Slice into 3/4 inch thick pieces and place the shrimp and basil on the plate with the terrine. Spoon some of the basil rich sauce over the top.