For the fennel juice, mix all of the ingredients together.
For the fennel cloud, line a freezer-proof dish with baking paper. Dissolve the caster sugar and 100ml/3½fl oz water in a pan, take off the heat and melt the gelatine leaf into it.
Whisk the fennel juice and albumina by hand, then add to a mixer with a whisk attachment. Whisk in the machine on a low speed gradually getting faster. When it has nearly tripled in volume, slow the machine down and add the sugar, gelatine and water solution. Whisk on high until stiff peaks are formed when the whisk is removed.
Pour in the Pernod, whisk through and then pour the fennel cloud into the prepared dish. Spread the cloud flat and sprinkle with the fennel pollen, if using. Put in the freezer for 6 hours, or until frozen.
For the tar-tar, mix the chopped asparagus and season with salt and pepper. Add a little olive oil and lemon juice. Mix in the fennel, add the tarragon and set aside.
For the green asparagus, in a hot pan, add the butter and the asparagus. Fry until the asparagus is nearly cooked and then squeeze in a little lemon juice and the beef stock. Cook for a further 2 minutes.
Spoon the green asparagus on to one side of a serving plate and spoon the tar-tar on the other side. Spoon the cloud on top of the tar-tar and garnish with the watercress. Serve immediately.