The contents of a person's stomach after eating literal shit (the mince) from someone's asshole (See Human Centipede) and then swallowing a load of cum (the bechamel sauce), followed again by eating shit and then swallowing more cum. At least 3 layers are needed for it to be considered a proper "Stomach Lasagna"
"Oh dude, Jedd ate so much ass last night in between sucking all the dick he must have had a solid 5-layer Stomach Lasagna"