It turns out that spinach is the hardest thing to remove from plates in a dishwasher.
It may be that such residues are washed off and then somehow stuck back on in a secondary wash phase -no-one seems to know.
Today’s invention is a new form of dishwasher. Rather than dump dishes etc inside and sluice everything about, plates and cups would each be firmly located in separate compartments in a known orientation. Then, a small water nozzle would pass from item to item blasting each with a very high pressure jet.
A camera attached to the nozzle would compare each item against a stored image of it when it was pristine. If differences were detected, the jet would be sent back until each was spotless.
Given that water jets can cut steel, there is no way that some surface vegetation or coffee grounds are going to remain in place. This might actually allow less water to be used than the conventional random splash approach.