It seems that most people using SPI are using Arduino as a master. I need it as a slave. There's a very useful page on Nick Gammon's site (http://www.gammon.com.au/spi) which helped me getting everything working somehow.
But then, at the end of the page I see that since Arduino 1.6.0 many functions are deprecated, namely:
This basically makes slave mode no longer supported, since it doesn't seem I can use beginTransaction() for that, or am I wrong? How do I make my code future-compliant?