Squeeze the lemon through your fingers into a mixing bowl, catching the pips as they come out. Add three times as much olive oil to the lemon juice and season well with sea salt and freshly ground black pepper. Whisk well.
Core and thinly slice each apple. Toss with the fennel and walnuts in the dressing and divide between smaller bowls. Crumble the Gorgonzola over the top of each, then scatter with the fresh peas and the leafy fennel tops to serve.