Cut the onions into small cubes.
Chop the fennel leaves / dill.
Soak the gelatine leaves in cold water.
Heat up the apple juice and stir in the gelatine leaves.
Add the apple juice to the crème frâiche and mix well.
Add the other ingredients and mix until even.

If you are making the herring tart, grind approx. 8 slices of Malax Loaf into crumbs in a food processor and moisten with apple juice.
Press the crumb mixture firmly down all over the base of the spring-form tin.
Spread the mousse mixture over the crumb base and allow to set in the fridge for approx. 3 hours.
The mousse mixture should be fully set.

If you are making the mousse, place the mousse mixture in a bowl and allow to set in the fridge for approx. 3 hours until the mixture is fully set.
Garnish the mousse or tart as you like, with roe, for example.