Fry the bacon gently in the olive oil for 5-7 minutes until crisp, stirring frequently.
Meanwhile, separate each liver into its two lobes, cutting away the white connecting tissue and removing any discoloured areas. Cut each lobe in half. Divide the spinach and watercress between four plates and sprinkle with salt and pepper.
Push the crisp bacon to one side of the frying pan and fry the chicken livers for 5-6 minutes over a medium heat, turning often, until brown all over but with a hint of pink in the middle.
Divide the liver between the salad plates. Add the vinegar to the pan over the heat and let it sizzle for a few seconds, then remove from the heat and add the nut oil. Add pepper, then spoon the dressing, including bacon bits, over the liver and salad. Sprinkle with grapes and serve with crusty bread.