Make sherry out of mead? Why not, some do that with wine.
It is a fermentation that is partly aerobic. It is one of the few cases when wine is aerated on purpose. A yeast that will produce a flor must be used (saccharomyces fermentati sold for instance by Red star).
After a first fermentation as usual, the must is pitched with the flor yeast and exposed to air. The mead must have reached 14 or 15 % alcohol at the end of the first fermentation: if less the exposure to air would be a suicide because of bacteria, if more the yeast may suffer. In some cases (nobody knows why it sometimes happens and sometimes not) a flor will develop, that is a thin solid layer will form at the surface. Wines having such a flor are thought to be better. The time of exposure depends on the taste one is looking for.
Spanish use the solera system: they us numbered barrels and every year, they bottle half the content of the first barrel and pour half the content of the second barrel into the first one, half the third barrel into the second one, etc. They use the wine they have just fermented to fill the last barrel. So the new wine "learns" from the older one. If there are ten barrels, the wine bottled is at least ten years old, but a priori some of the wine is as old as the winery!
References: chapter 5 in V4 (Knap, Cooper, Roesener), chapter 9 in V3, chapter 9 of V6.