Blend the tomatoes with the onion in a food processor to produce a fine pulp. Strain the pulp through a fine-mesh strainer into a large bowl.
Lightly fold in the mayonnaise, sour cream, heavy cream and tomato puree into the tomato pulp. Add a pinch of salt and pepper and the finely chopped chives and fold into the mix.
Place the gelatin into a small pan and add the lemon juice. Melt the gelatin over low heat until it dissolves, then pour it into the tomato mix—stirring as you pour.
Adjust the seasoning with salt and pepper, to taste.
Pour the mixture into individual ramekins or a mold and refrigerate for at least 2 hours.
Just before serving, run a small knife around the edge of the mold and turn out the mousse onto a plate.
Slice and serve.