Lightly grease a 9-inch pan, or line it with baking parchment.
Boil the sugar and water together in a medium pan. Stir lightly an reduce the heat. Cook the syrup 15 minutes, or until it coats a spoon thickly. Don’t stir the syrup, after it comes to a boil, just let it cook. Take it off the heat and let it cool a few minutes.
In another pan, heat the tahini until warm (not hot), stirring.
Pour into a bowl and add cinnamon, cloves, lemon juice and vanilla extract. This can be done by hand, in a bowl, using a strong spatula. Add the sugar syrup and mix quickly; if left in the bowl more than a few minutes, the halvah will set. Pour it into the pan and press down to get rid of any air bubbles.
Allow the halvah to cool down completely, then cover it with plastic wrap and refrigerate for 24-36 hours. Cut it into slices or cubes while still cold, but it’s best eaten at room temperature.