Preheat the oven to Gas Mark 3, 160ºC (325ºF). To prepare the lamb, place the olive oil, ginger, cumin, coriander, turmeric, lemon rind and juice, the stalks of the coriander and a good pinch of salt into a blender and blitz to form a smooth paste. Put the shoulder of lamb onto the two large pieces of foil and make lots of incisions with a knife. Next rub the paste all over. Add a couple of tablespoons of water then seal the foil leaving a little pocket of air above the lamb. Place onto a baking sheet and cook for 3 hours.
After 3 hours open the foil and baste the lamb, then reseal the parcel. Cut the tips off the tops of the garlic cloves, wrap the bulb tightly with foil and place in the oven. Cook the lamb for another hour until the lamb is meltingly tender and the garlic bulb is soft. Turn off the oven but leave the lamb to rest inside.
Meanwhile, make the salsa. Place the tomatoes in a bowl with the red onion, herbs, lime juice and olive oil. Season to taste and mix thoroughly to combine.
To make the roasted garlic aioli, gently heat the olive oil. Squeeze out the really soft insides of the garlic cloves into a blender. Add the egg yolk and some salt and pepper. With the motor running, slowly add the olive oil to make a smooth thick sauce. Place into a heavy-based pan so you can heat through before serving (this could also be done with a small whisk and a bowl).
Heat the tortillas in the oven by wrapping in a clean damp tea towel. Shred the lamb using two forks, discarding any excess fat and bones. Give each person a tortilla, dot over the garlic aioli, top with some lamb, a good spoonful of the salsa, then finish with some salad and a couple of slices of the cucumber. Wrap and eat immediately. Serve with the green salad.
Note: If you use a boned shoulder you could reduce the cooking time by approximately an hour. You may have some lamb and aioli left over, they will keep in the fridge for a couple of days.