Bring the stock to the boil, simmer the peas for approx. 4 mins., mix with lemon juice, leave to cool. Puree the peas with 2 tbsp of cream until smooth. Transfer to a whipped cream maker along with the remainder of the cream, shake well, refrigerate for approx. 1 hr.
Mix the flour and salt. Beat the egg lightly with the water, add to the flour mixture along with the olive oil. Combine with a risotto spoon and knead to form a smooth dough. Leave to rest for approx. 30 mins. under a bowl rinsed in hot water.
Heat the butter and sauté the onion. Add the mushrooms, season. Sauté the mushrooms, covered, for approx. 5 mins. Add the cream and cognac, reduce the liquid and allow the mushrooms to cool.
Then, on a lightly floured surface, roll out the dough as thinly as possible into a rectangle and place on a cloth. Dust your hands with flour and carefully stretch the dough until it is approx. 45 x 55 cm in size, brush with butter, spread the filling over the top, leaving a gap of approx. 2 cm all the way around. Fold the pastry ends inwards. Roll up the strudel, using the tea towel to help you. Transfer the strudel to a baking tray lined with baking paper. Brush with butter.
Bake for approx. 25 mins. in the centre of an oven preheated to 200°C. Serve with the pea mousse.
Pea foam (version that does not use a whipped cream maker): Beat the cream until stiff, carefully fold into the lukewarm pea puree.