After its defeat in 1806, Prussia lost almost all the lands it had gained in the Second and Third Partitions of Poland.
The Congress of Vienna would see Prussia regain much of its Second Partition lands, but the Prussian portion of the Third Partition went to Russia.
In exchange, Prussia received Westphalia & the Rhineland.
Your challenge, should you choose to accept it, is to craft a situation, with a POD after the Treaty of Tilsit, wherein Prussia regains the entire of its 2nd and 3rd Partition lands at the Congress of Vienna.