Heat water in a saucepan and dissolve in Marmite®. Turn down heat just before it boils and drizzle in tahini until the sauce thickens, stirring all the time. Adjust the quantities to taste; you can add more tahini for a thicker sauce. Serve it just like that.
Tahini can be expensive, so for larger quantities use cornflour to thicken the gravy. You can use Vecon® instead of Marmite®, or cow's milk to replace some, or all of the water. Depends what you're serving it with.