Make the dressing: In a blender blend together the tahini, the lemon juice, the garlic paste, and the cayenne, with the motor running add the oil in a stream, blending until the dressing is emulsified, and season the dressing with salt. Make the tuna salad: In a bowl toss the tuna lightly with half the dressing and mound the mixture on a large platter. In a large heavy skillet cook the onions in the oil over moderate heat, stirring occasionally, for 50 minutes, or until they are golden brown, and season them with salt and pepper. With a fork scatter the onions over the tuna. In the skillet cook the pine nuts over moderately low heat, stirring, until they are golden and scatter them over the onions. Drizzle the salad with the remaining dressing and sprinkle it with the chopped parsley. Garnish the salad with the parsley sprigs and serve it with the pita pockets.