Preheat oven to 400. Slice the tops off the garlic and place the bulbs on a sheet of foil. Spoon over 2 tbsp of the oil and sprinkle with salt. Wrap the foil over the garlic and bake for 1 hour, until soft. Open out the foil and leave the garlic to cool.
Discard the crusts from the bread. Soak the bread in the water for 1 minute, then squeeze dry and place in food processor. Squeeze the garlic flesh into the processor. Process to a smooth paste.
Add 2 tbsp lemon juice with a little salt and pepper. With the machine running, gradually add the remaining oil in a thin stream to make a smooth paste. Check the seasoning, adding more lemon juice if needed. Turn into a bowl, cover and chill until needed.