Wash the grapes and pull them off the stems. Put them in a 6-quart / 5.7 L stockpot with the 250 ml water.
Crush the grapes a little with a wooden spoon, then bring to a gentle boil over medium heat. Simmer for 15 to 20 minutes, stirring now and then, until the grapes are soft and pale.
Set a fine-mesh sieve lined with cheesecloth over a large bowl. Pour the grapes and their liquid through, then press with the back of a spoon to squeeze out as much juice as you can. Discard the skins and seeds.
Measure the strained juice, you should have about 2 liters. Pour all but 1 cup (240 ml) of it back into the cleaned stockpot.
Whisk the flour into the reserved 1 cup of juice in a small bowl until completely smooth, with no lumps.
Bring the juice in the pot to a gentle simmer over medium heat.
Slowly pour in the flour slurry while whisking constantly, this keeps the pudding smooth.
Cook for 20 to 25 minutes, stirring often with a wooden spoon, until the pudding thickens enough to coat the back of the spoon and pulls away from the sides of the pot when you stir.
Pour the hot pudding into the 23 x 33 cm / 9 x 13-inch dish and smooth the top with a spatula.
Press the walnut halves into the surface while the pudding is still warm, spacing them evenly. Dust with the cinnamon if using.
Let the pudding cool at room temperature for about 1 hour, then cover and refrigerate for at least 4 hours, or overnight, until fully set and sliceable.
Slice into squares or diamonds and serve chilled or at room temperature.