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Melt the butter in a heavy-based pan. Once it’s foaming, add the asparagus and cook for 4-5 mins until soft but not coloured. Remove from the pan and drain well on kitchen paper. Rinse out the pan and pour in the cream, milk and garlic. Bring to the boil, simmer until reduced to 350ml – check by pouring into a measuring jug – then add the asparagus and boil for a few secs. Remove from the heat.
Blend the mixture in a liquidiser along with the spinach leaves, then sieve into a clean bowl. Add the soaked gelatine and stir until dissolved. Sieve again and allow to cool for a few mins in the fridge, but do not let it set.
Beat the egg white to soft peaks, then carefully fold into the asparagus mix and season. Divide the mousse between 4 glasses or ramekins and leave them to set in the fridge.
To make the ham salad, mix all the ingredients, apart from the watercress, in a small bowl and season. When the mousse is set, remove each glass from the fridge, place some watercress leaves on top of the mousse, spoon on the diced ham salad and drizzle with a little olive oil.