It’s the kind of idea you can’t believe no one had carried out before.

“Farm to table” is still the buzz nationally for restaurants, and New York City has more than 4,200 of them. There’s not much space for farms here, though. So why not put a mini-farm on the rooftop?

Restauranteur Mick O’Sullivan found out the answer in 2010: regulations. He wanted to install a garden atop Bell Book & Candle, his cozy West Village dining spot with chef and business partner John Mooney. They soon discovered that New York City wasn’t zoned for it.

After calling office after office, and eventually hiring a lobbyist type, O’Sullivan finally got the garden allowed. They set up a hydroponic system. Towers of plants use water, not dirt, to nuture growth through a gravity-fed reservoir system. O’Sullivan tends the 60 towers himself, growing herbs, lettuces, and tomatoes in abundance.

The garden produces for a long season, from April through December (72-degree water combats the cold air for hardy veggies like cheddar cauliflower). I visited yesterday to see the towers being set up again. I tried a chive, grown from seed by a Pennsylvania Dutch farmer and about to be transplanted into the towers. It tasted just like chives from my parents’ garden.

Besides saving the world and all, there are other advantages to a rooftop garden, O’Sullivan says. One is long-term cost savings. The other is control. “The quality and taste are second to none,” he says. “Our tomatoes never see a fridge.”

It’s too early to be eating from the rooftop yet this spring. But dining with friends Caitlin McRobbie and Emily and Tyler Cleland after the tour, I found that Mooney makes excellent homestyle food even with ingredients sourced from beyond upstairs.

The oozy grilled cheese, made with a Swiss grilling cheese called raclette, black truffle, and soft-boiled egg, was a standout. We also loved the BB&C burger, finished with a housemade pickle, and the deep-black, cheese-topped Irish American onion soup. As an Irish American myself, I say, why should the French get all the glory?

The surprising thing is that not more than a few other NYC restaurants have set similar systems up since O’Sullivan and Mooney cut through the red tape five years ago. At $500 per tower, the garden isn’t expensive, either. Here’s hoping more places will follow suit.