In a medium skillet, heat oil over medium heat; add celery and onion and cook, stirring occasionally, until softened, about 5 minutes.
Add the tomato wedges, cover and cook for 15 minutes, stirring once or twice; uncover and cook another 10 minutes so that some of the moisture evaporates from the tomatoes.
Press mixture through a strainer into a medium bowl; discard solids.
Return the strained mixture to the pan, reduce heat to medium low and heat through (should be the consistency of tomato sauce - add water if needed).
Combine the gelatin with hot water in a small bowl and stir until dissolved.
Whisk cream in a separate bowl until slightly thickened.
Stir in tarragon, dissolved gelatin, and whisked cream to the tomato mixture; season with salt and pepper.
Pour mixture into a 4 cup terrine or gelatin mold; alternatively the mixture can be divided among individual martini glasses.
Refrigerate for 8 hours, or until set.
To serve, run a knife around the inside edge of the mold; immerse the bottom of the mold in hot water for 30 seconds.
Place a plate over the top and invert so the terrine unmolds onto the plate.
Cut into slices to the desired thickness; transfer to individual salad plates.
Alternatively, serve filled martini glasses on individual plates.