This is more a concept than a recipe. Have deep pottery or porcelain crocks or glass jars -- no metal containers. A small container would be two cups; a large one two quarts. Wash with hot, soapy water, rinse with boiling water and dry. Make a layer of onions thick enough to cover bottom of container. Completely cover onions with brown sugar. Sprinkle about 8 to 10 tarragon leaves and a little salt. Continue layering the same way, occasionally pressing down to compact, until container is full to within 1-inch of top. End with the sugar layer. Pour in enough vinegar to cover. Cover with a lid not crewed tight, or loosely with plastic wrap (not foil) and store in a cool, dark place. To use, pull onions out of crock, pile in a shallow bowl. Use atop cheese slices or as suggested above.