For the past three months, Mark Bittman has been running a curious experiment: a fine dining restaurant where patrons pay as little as $15 for a meal. The former New York Times food writer wanted to see whether it was possible to run a restaurant where the food was healthy and locally sourced, the staff was treated and paid well and the prices were affordable. His solution: philanthropic donors, and prices on a sliding scale. As Community Kitchen prepares for its final dinner service on Saturday, Bittman tells Matt Galloway about what the project has taught him about what it would take to change the food system.