In a small bowl, whisk together the lemon juice, garlic and salt, leave this to sit for a few minutes, then whisk in the olive oil, and fold in the Roquefort and crème fraîche.

Put your prepared spinach into a large bowl and heat up a skillet for the pancetta.  Cook the pancetta for several minutes until very crisp, use a slotted spoon to transfer it to a paper-towel lined plate, then add the grapes to the pancetta fat in the still warm skillet.  Cook the grapes for a couple of minutes until they begin to swell and split, stirring in splashes of sherry vinegar as needed.

Spoon the grapes and all of the juices over the waiting spinach, add the dressing a little at a time and toss to combine well.  When you feel you have enough dressing, serve the salad with the cooked pancetta scattered on top.