Peel and finely slice the onion. Drizzle a little olive oil in a non-stick frying pan and place over a medium-low heat. Sauté the onion for 10 minutes, or until soft and sticky but not coloured.
Add the raisins to the pan and cook, stirring, for 1 to 2 minutes, then add the vinegar. Take the pan off the heat.
Crumble the cheese into a blender and add the juice of the lemon. Pour in a few tablespoons of extra virgin olive oil, season well, add a splash of water and blitz into a smooth, creamy dressing.
Place a griddle pan over a high heat and leave for a few minutes to get scorching hot.
Trim and remove the tatty outer radicchio leaves, then cut into 3cm wedges and cook, cut-side down, in batches on the hot griddle pan for a few minutes on each side.
Toast the pine nuts in a dry frying pan.
Remove the radicchio to a bowl, spoon over the onion mixture and scatter in the toasted pine nuts. Toss until well combined.
Transfer to a platter or board and drizzle over the Gorgonzola dressing. Finish by scattering over the rocket leaves, then crumble over extra Gorgonzola, if you like, and serve straight away.