On a recent evening, a hip-looking young woman was sorting through a stack of crates outside a fruit and vegetable store here in the working-class neighborhood of Vallecas as it shut down for the night. At first glance, she looked as if she might be a store employee. But no. The young woman was looking through the day’s trash for her next meal. Already, she had found a dozen aging potatoes she deemed edible and loaded them onto a luggage cart parked nearby. “When you don’t have enough money,” she said, declining to give her name, “this is what there...