long a staple什么意思（有全文）

全文如下：
The ladies were puzzled. Cheryl Spangler, Valeria Borunda Jameson and Susan Puckett, three university-admissions workers on a training wisit to Florence, Kentucky, had walked into a local barbecue joint called Chung Kiwha. But instead of sauce-covered mutton served up from the kitchen, they saw a buffet of uncooked meats and vegetables. Instead of knives and forks, they were given large scissors, chopsticks and metal tongs. No candle flickered at their table, but a bucket of fiery wood charcoal hissed in the tabletop grill pit. Chung Kiwha served barbecue, all right-cook-it-your-self Korean barbecue. "I didn't realize there were restaurants like this," marveled Spangler to her friends, who hail from Knoxville, Tennessee, and I worked in restaurants for 20 years.

The secret is out, thanks to the growing popularity of restaurants where the customer is the chef. Long a staple of immigrant communities in big cities, restaurants where diners chop, grill, boil, or dip their dip their food are hot in the American heartland. St.Paul, Minnesota, has Thai hot-pot cooking. Indianapolis, Indiana, has Japanese shabu-shabu (another type of hot pot). A pizzeria in Las Vegas lets customers roll the dough.

那么long a staple那个部分是什么意思呢
是个什么结构

个人意见：
Long a staple of immigrant communities in big cities 作状语，修饰主句restaurants are hot in the American heartland.

相当于As a staple of immigrant communities in big cities for a long time

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