Arduino for STM32

Which example? Both of the examples on Github under the SPI section seems to have the STM32 as an SPI master, not slave. My goal is that I want to have two STM32 devices communicate with each other using SPI.

I'm currently using the official STM32 core and I want to know how I can make my chip, the STM32F303 Black Pill act as an SPI slave?