Brush 6 dariole moulds (100 ml/ 31⁄2 fl oz capacity) with mild olive oil and line the bases with discs of baking paper. Shell the peas and trim away and discard the cut end of the asparagus. Chop the asparagus into short lengths and put in a steamer with the peas. Steam for 3-5 minutes until the asparagus and peas are just tender but still bright green. Put 6 tbsp of steamed peas into a small bowl, spoon over the 1 tbsp olive oil and set aside.
Put the asparagus and the rest of the cooked peas in a food processor and whizz to a purée. Soften the gelatine in the wine or lemon juice in a small pan for 5 minutes, then put over a low heat until the gelatine is dissolved, stirring occasionally.
Whisk the cream with the tarragon until it is thick but still glossy. Stir the gelatine into the purée and fold in the cream with plenty of salt and pepper. Divide the mixture between the moulds so they are three quarters full, then put in the fridge for 2-3 hours or until set.
When ready to serve, run the blade of a small knife around the rim of the mousse and turn out each one on a small plate. Sprinkle the reserved peas around the base of each mousse and lay a tarragon sprig on top of each one. Drizzle a little more olive oil and a few dashes of balsamic vinegar over the peas. Serve straight away while still chilled.