Combine mushrooms, celery and Dijon Mustard Dressing.  Marinate at least 1 hour; stir several times.  Add 3 tablespoons pistachios; mix well.  Remove top of tomatoes; scoop pulp out of centers.  Drain tomatoes upside down on paper towels.  Spoon about ½ cup mushroom mixture into each tomato.  Serve on lettuce-lined platter; garnish with 1 tablespoon pistachios. 

Dijon Mustard Dressing: 

Combine 2 tablespoons oil, 1 tablespoon lemon juice,

¾ teaspoon Dijon mustard, ⅛ teaspoon grated lemon peel, dash each salt, pepper and crushed tarragon; mix well.