I garden, and one of the main reasons I do is to grow vegetables and herbs that can't be got at the store, except maybe on rare occasions and even then they are usually crap.
But many are quite easy to grow, and for a restaurant specializing in unusual stuff grown locally, would be worth working with:
kohlrabi - fabulous raw or cooked
mustard greens, varying from mild to spicy
mizuna
sorrel
leafy red lettuces (my favorite variety is something called merlot lettuce)
red romaine lettuce
arugula
golden beets
multi-colored swiss chard
chervil (a wonderful sweet herb that doesn't keep, and so will never be found in decent condition in a grocery store)
cilantro, oregano, greek oregano, dill, varieties of basil (you can find them at the grocery, but the fresh stuff is vastly superior).
chives and garlic chives, which are perennial and come back every year. The fresh stuff, again, is far superior to what you can buy.
These can all be grown in a relatively small garden, with ease. I once ate at a Bay area restaurant south of Hayward, CA, which had its own garden for such stuff, and it was fabulous. It's a great idea if you want to play around with a foodie-style fiction plot. Remember, Rex Stout made great mileage out of Nero Wolfe and his obsession with gourmet food and orchid-growing.
caw