From the Kuenstlichs und Fuertrefflichs Kochbuch again:
19 Filled Semmel Loaves (Gefuelt Semmel)
Take a white Semmel loaf, slice off a thin lid on the top and hollow out the crumb. Then take little birds that go on skewers (Spiesvoegelein), boil them and fry them in fat. Take Rosinlein and Weinberlein (types of raisins) and blanched almonds, fill the Semmel loaf with that and place the lid on top again. Thus it fits together as though it were whole again. Prepare a batter as if for Streublein and place the Semmel in that. Pour the batter all around with a spoon. Take fat and put it into a pan. Let it get hot, then take the Semmel out of the batter and put it into the pan. Pour a spoonful of batter over the lid on top, quickly pour fat over it and see none of the batter gets into it. Fry it nicely, and spoon fat over the top, thus you do not see the Semmel. When it is fried, take a bowl and pour in a sauce (pruee) of wine, add sugar, also ginger and pepper, and a spoonful of fat. Let that boil, take off the lid again, pour in that sauce. Thus the weinberlin swell up and it is done.
My guess is that this was what you did if you wanted to have pastries, but did not have the time, equipment, or ingredients to prepare proper pastries. A semmel, basically a small breadroll similar to a manchet loaf, could be bought any day and while the kind of birds small enough to fit one might not have been available at all times, they were hunted year round. I am also fairly sure that other meats would go into similar preparations. They certainly do today, as a number of recipes for gefüllte Semmel with ground meat or scrambled eggs demonstrates. The only thing that surprises me a little is that you would open the lid after going through such trouble to conceal it exists, but pouring in a hot liquid was what you did with a lot of meat pastries at the time.
The short Kuenstlichs aund Fuertrefflichs Kochbuch was first printed in Augsburg in 1559 and reprinted in Nuremberg in 1560 and subsequently. Despite its brevity, it is interesting especially as it contains many recipes for küchlein, baked or deep-fried confections, that apparently played a significant role in displaying status. We do not know who the famous cook referenced in the title may have been or if he ever existed.