Cut the pomegranates in half, squeeze it into a bowl, then remove the seeds. Discard any pith. Set 2 Tbsp seeds aside.
Place the rest of the seeds and juice in a food processor and blitz until you get a purée. Strain it through a sieve, pressing the pulp with a spoon.
Put the strained pomegranate liquid in a saucepan with the cream and sugar. Add the lemon zest and juice too.
Bring to a boil, then reduce the heat and simmer gently for 5 minutes.
Let it cool. Using an electric whisk, whip it until light and fluffy.
Spoon the mousse into two glasses and chill for a few hours or until set. Alternatively, put in into the freezer for about an hour.
Scatter the reserved pomegranate seeds on top just before serving.