Give my wife time, she'll show those damned Brits how to make a Fatburg!
That's exactly what it was, too: cooking grease.
It looked like someone dropped two dozen bars of Ivory soap down the drain and then they got broken up when I dislodged them but nope, that's not soap, that's cooking grease.
There's no changing her behavior so I guess I'll have to look into grease traps like they would use in a commercial kitchen.