With Ethel Kennedy on hand to ogle the onions, it wasn't the everyday supermarket crowd that turned out in McLean last night to celebrate the opening of the area's newest gourmet food store.

The exotic food preserve sells everything from Wild Boar ham steaks to Grand Marnier ice cream, and some of Fairfax County's finest lined up in the cedar-lined store to gobble and goggle.

"The onions!" exclaimed Kennedy, one of McLean's more celebrated hostesses. "I plan to shop here a lot," she said, as her sister-in-law, Eunice Kennedy Shriver, nodded in agreement. "We won't have to go over the bridge to Georgetown anymore," she said.

The 300 who showed up to sample the wares at Giant Gourmet seemed eager to welcome what some described as the gastronomical equivalent of Bloomingdale's to the heart of downtown McLean. And those in charge of the store were clearly delighted. "We're not just Bloomingdale's," boasted Giant Food Inc. president Israel Cohen. "We're Neiman Marcus, I. Magnin, and Lord and Taylor, too."

Gourmet stores are part of a move by supermarket chains to target their customers as competition grows keener and specialty food outlets proliferate. Safeway and Giant, the two largest food retailers in the area, both already have no-frills grocery operations for the shopper bent on economy. With last night's opening, both now compete for the "upscale" shopper on the lookout for exotica as well.

"This is the ideal location for this kind of store," declared Giant vice president Gerson Barnett. McLean is the most affluent area in one of the nation's most affluent counties, with the added virtues of good access from the Maryland suburbs and the District of Columbia.

The Gourmet Giant doesn't sell saltines, nor will you find paper towels, laundry soap or scouring pads. It does have seven sorts of melons, two kinds of carrots, six mushroom varieties, Peruvian shrimp, Italian white artichokes, caviar that sells for $223 a pound, bottled water that costs more than some wines.

"Fresh figs!" exclaimed a French photographer.

You won't find the National Enquirer at the checkout line. Instead, there are five different gourmet cooking magazines; no Mars Bars and chewing gum, but several varieties of imported Dutch chocolate, at $8 a pound. And when they ring up the bill on the computerized, laser checkout machines, you can charge it to American Express or three other cards.

The new store occupies what once was an A&P, and its 23,000 square feet are right across the street from a regular Giant. At least one Giant competitor, the Magruder's supermarket chain, was interested in the A&P site, but, according to Magruder's president Louis Fanaroff, Giant scooped up the site first.

"This isn't the elite, this isn't all about the jet set," said Giant Food coordinator Ann Brody. "We're fulfilling a need. People are more interested in food now. Food is culture, life, religion."

Last night about half of those who showed up to sample the culture, life and religion were mink-swathed matrons nibbling on pasta salads, and men in black tie wandering the store's aisles balancing paper plates full of pate' and roast meats. Former Virginia GOP gubernatorial candidate Marshall Coleman stopped by to sample the cheese and Fairfax County Supervisor Tom Davis showed up to grab a free dinner.

About the only creatures not eating were the 60 wriggling rainbow trout, shipped in tanks from trout farms in North Carolina, and the lobsters in adjacent holding tanks back in the seafood department.