1 bunch of celery, separated into ribs, washed well, trimmed, and strings
1/2 pound Gruyere
3 tablespoons pine nuts
1/2 cup packed fresh basil leaves
2 tablespoons white-wine vinegar
1/3 cup olive oil
With a small knife trim a strip down the length of the rounded side of each celery rib to form a flat bottom and cut the ribs on the diagonal into 1 1/2-inch boats. Put the celery boats in a bowl of ice and cold water and chill them, covered, until ready to assemble the hors d'oeuvres. Cut 6 ounces of the Gruyere into 1-inch-julienne strips and grate fine the remaining 2 ounces. In a food processor or blender combine the grated Gruyere and the pine nuts and blend the mixture until it is minced fine. Add the basil and the vinegar and blend the mixture until it is a paste. With the motor running add the oil in a stream and blend the pesto until it is emulsified. In a bowl toss the julienned Gruyere with the pesto and fill the celery boats, drained and patted dry, with the mixture. The hors d'oeuvres may be made up to 1 day in advance and kept covered and chilled. Serve the hors d'oeuvres at room temperature.