Prepare fish by rinsing it, then patting it dry. Make 2 to 3 diagonal cuts on the side of the fish (with the blade of the knife on an angle facing the head). (source)
The preposition phrase "on an angle" strikes me as strange. I have always taken "at" as the most common preposition that goes with angle. I found another sentence using this phrase on Merriam Webster.
The road goes off on an angle. (source)
This NGram chart shows "at an angle" is decidedly more common than "on an angle". What does "on an angle" mean exactly in the two sentence at issue? Is it the same usage in the sentences above? How does it differ from "at an angle"?