I have a problem understanding this verbal confrontation:
In the Black Earth Rising series, a lawyer is hired by dangerous men, represented by a priest. But it seems that they refuse to pay for the service, while the lawyer demands the payment or he will turn against his former client. At the end of this conversation we can hear this:
Lawyer: Pay the piper, Padre. Or I'll pipe the rats right back.
Priest: Actually, I'm a pastor.
Lawyer: Yeah? Then go boil some milk.
The first sentence is an obvious reference to the fairy tale about Pied Piper of Hamelin, in which a piper uses magical flute to lure the rats out of town, but the citizens refuse to pay him, so he uses his magic on their children taking them away too. So the quite obvious meaning is "pay for the provided service or face the dire consequences".
The quick google search for "pastor" + "boiling milk" only shows the Bible fragment about "not boiling a young goat in its mother's milk", but I'm not sure how this is relevant to the plot.