I would be worried that unscrupulous restaurants might use the leftovers that come back into the kitchen from peoples` tables. Or the unused portions of curry left in the balti dishes on the table.
I saw an undercover documentary where a restaurant rinsed off the uneaten bits of meat, put them in whatever sauce a new customer had ordered and sent it back out.
I would imagine the kind of food that will be sold off would be stuff that has been prepared or even cooked. The steaks would probably go back into the fridge for the next day.
Most commercial waste food goes in to anaerobic digesters now ,where it is turned into Methane. The Methane is then burned in gas fired power stations to produce electricity. This is a good idea for small portions of cooked food.