In a small frying pan over a medium heat, lightly toast the pine nuts until they're just beginning to turn golden, then tip out into a food processor. Add the garlic, sorrel, parsley and a pinch of salt to the pine nuts, then pulse a few times until roughly chopped and combined. Slowly pour in the olive oil, pulsing as you go, until the pesto is the consistency you like.
Spoon the pesto mixture into a bowl and stir in the goat's cheese. The pesto will keep, sealed in a jar with a slick of olive oil over the top, for about a week.