Want a glimpse and a taste of how the rich eat? At the medieval fair at Trinity Episcopal Church in Upperville, Va., on Saturday one of the booths will be manned by a "top chef from the Mellon estate." He will demonstrate the art of making omelets and prepare them for sale to guests who gain admission by paying $2, or $9 to attend the Hunt Country Fashion Show as well as the fair. Proceeds go to charity. The fair begins at 10 a.m. The fashion show is scheduled for 3 p.m.

Those with a yen to go into the country to find food rather than fashion might consider the Fountain Rock Springs Trout Farm, located at Walkersville, Md., just north of Frederick. The farm supplies trout to a number of local restaurants, but also offers would-be anglers the nearest thing to a sure bet they are likely to find this side of a fish store.Admission is $1. You bring rod and reel, but there is no need for a license, no limit and the management will gut and clean the fish. Brown trout went $2.39 a pound on Labor Day and miniature marshmallows worked out just fine as bait. Owners Tom and Patricia Denike are planning to stock coho salmon as well in the future. For information and directions call (301) 898-5335. The farm is open every day from 7:30 a.m.

Looking farther afield, with an eye on cooking and eating, Johnson & Wales College of Providence, R.I., a leader in the culinary education field, is offering a "Cook'n'Tour" program on nine weekends this fall. The cooking feature is two classes with the school's chef-instructors in preparing haute cuisine entres and deserts. Dining events include two dinners and a Sunday brunch. Lodging is provided by the college. Also on the two-day, two-night schedule is visit to Sakonnet Vineyards in Little Compton, R.I. The cost is $165 (double occupancy) or $180 (single occupancy). For further information and reservations write Cook'n'Tour Weekend, Johnson & Wales College of Continuing Education, 8 Abbott Park Pl., Providence, R.I. 02903, or telephone (401) 456-1120. The final weekend of this series begins Nov. 7

A limited number of tickets still are available for an unusual event combining music and food. The Armenian General Benevolent Union is honoring Miran Kojian, concertmaster of the National Symphony Orchestra at a reception following a Sept. 28 performance by the orchestra's string quartet at the Cocoran Gallery of Art. Half-a-dozen specialty foods, prepared by members of the local Armenian community, will be served at the reception. The concert begins at 8:30 p.m. The reception follows it. Requests for tickets, at $15 per person, should be sent to A.G.B.U. -- Washington Chapter, c/o Doris George, 4151 Elizabeth Lane, Annadale, Va. 22003. Please enclose a stamped, self-addressed envelope.

Chicken fanciers should consider a nifty softcover "book" called "Chicken."

I write "book" because it is only 32 pages long and is printed in an 8 1/2-inch-by-11-inch, softcover format. Nonetheless, the author, Sally Crowder, has contributed a series of instructive tidbits about the chicken, nearly two dozen recipes (several of them from the Diet Workshop) and a wonderful series of antique engraving showing various breeds of chicken. The book is worth considerable more than any number of tomes several times its weight. It is available at all Kitchen Bazaar stores for $3.99.

The Dom Perignon of olive oils, Badia a Coltibuono, is available in Washington. Like the famous champagne, it may not actually be the best of its breed. But it does have considerable class, a distinctive bottle and it costs more than the competition. A lot more. Capitol Hill Wine & Cheese is selling this tall, distinctive, hand-blown, 1-liter bottle of vintage, extra virgin Italian oil from the Siena region of Tuscany for $22. A&A Liquors has plans to sell Badia a Coltibuono, too, but the first shipment arrived broken. It should sell there in the $17 to $20 range.

This is but the latest in a series of luxury food products that, by themselves, seem to be outrageously priced. The purchase is justified because the product is very, very good (which it is) and in absolute terms the price is not very great. Where else, the hedonists and chics-in-waiting ask, can you buy distinction and luxury for $20?

The fat lady will be eating instead of singing at the Capital Centre in Landover from Nov. 14 to 16. A new presentation, the Washington International Food Show, will be held on those dates. Planners expect at least 140 displays and 35 major exhibits of food preparation and display items. lThere also will be a marketplace for food shopping and an international food bazaar featuring food to eat on the premises. Cooking demonstrations are on the schedule, too. Tickets are available at all Capital Centre outlets and cost $4.50 for adults; $2.25 for children.