For lemon crème fraîche, process all ingredients in a food processor until smooth. Season to taste with sea salt and freshly ground black pepper.
Cook peas in a large saucepan of boiling salted water for 4-5 minutes until tender, then drain and mash coarsely using a fork. Whisk egg yolks until frothy, then stir in peas, green onions, mint, lemon rind, breadcrumbs and baking powder and season to taste. Whisk eggwhites until soft peaks form, then fold into pea mixture, in batches, until just combined. Heat 1cm oil in a large frying pan and cook heaped tablespoons of the fritter mixture, in batches, for 2 minutes each side or until golden, remove and drain on absorbent paper. Serve immediately with lemon crème fraîche.