LATE IN October and in early November, when the grape harvest begins in the Rhine and Moselle valleys in Germany, entertaining takes on a seasonal form. Invitations often say, "Come on over for a glass of wine and Zwiebelkuchen " or onion pie.

Within a week of the first gathering of grapes, vintners sell Federweisser or feather white, a sweet, milky white brew developed before fermentation is fully completed. Experts call it a cross between grape juice and wine. Signs pop up at street corners, in restaurant windows or on doors, announcing that it is the time for the Federweisser.

Wine connoisseurs frown a bit on the "impetuous, raging young wine," but to most people it's fun to sample the season's latest.

Because Federweisser is a rather powerful drink, substantial peasant fare is dished up alongside, the most traditional being onion pie. The sample yeast, dough, topped with crisp bacon and onions, has its roots in the wine-country of Alsace-Lorraine. But it spread to most German wine-growing regions, where each district boasts its own specialty today.

Suddenly the wine villages, quiet since the end of the wine festivals, experience a new rush of vistors. Hikers taking advantage of the last beautiful fall days roam the hills and revelers check out the vineyards: they stop intermittently for a repast of wine and onion pie.

Although the Federweisser can be bought by the pitcherful, most parties at home rely on fully matured wine, which is entirely different. More than two glasses of Federweisser would surely bring on the katzanjammer (hangover) the next morning.

For the accompanying onion pie German housewives have the advantage of being able to buy ready-made yeast dough at their bakers. Frozen bread dough will substitute nicely, if it has been taken in plenty of time to thaw and rise.

The following is a typical recipe from the Moselle area: ZWIEBELKUCHEN (Onion Pie) (8 servings) Yeast Dough 1 1/4 cups warm water 1 package yeast 1 tablespoon sugar 1 tablespoon shortening 1 1/2 teaspoons salt 3 1/2 cups flour Topping 6 slices bacon, diced 2 large onions, sliced 2 egg yolks 1 cup heavy cream Salt and pepper

Pour the water into a large bowl. Add yeast and sugar and still until dissolved. In another bowl, cut shortening into the flour and sprinkle on the salt. Gradually stir the flour mixture into the yeast, until a stiff dough forms. Transfer the dough to a floured board and knead until it is smooth and elastic, about five minutes. Place the dough in a lightly greased bowl and cover with a towel. Let the dough rise until double in bulk. Punch the dough down and roll it out into a rectangle of about 1-inch thickness. Place on greased baking sheet or pizza pan. Pinch the edges to form a rim.

Fry bacon until crisp. Drain on paper towels. Reduce the heat and cook the onion in a little bacon fat until softened and golden. Drain.

Blend the egg yolks with the cream.Season with salt and pepper. Spread the cream mixture evenly over the dough. Top with onion and bacon bits.

Bake at 400 degrees for about 30 minutes. Serve at once.