Bring the tomato juice, 250 ml water, stock powder, thyme, lemon slices, salt and pepper to the boil in a saucepan and simmer for 15 minutes over medium heat. Strain the mixture through a sieve into a mixing bowl.
Sprinkle the gelatin over the 60 ml cold water and leave for 5 minutes. Stir the softened gelatin into the tomato mixture, ensuring it has dissolved. Pour the mixture into a shallow ovenproof dish and evenly scatter the peppers, cucumber, chives and celery on top. Cool and refrigerate overnight.
Roughly dice the tomato jelly and spoon a little into the bottom of 10 glasses. Top the tomato layer with a spoonful of diced avocado and a teaspoonful of mascarpone. Repeat the layers once more, working carefully so the layers are clearly distinguishable. Refrigerate until ready to serve.