Using Arduino NANO Every as SLAVE in SPI communication

Hi,

i'm trying to establish an MASTER/SLAVE Communication between 2 Arduinos.

The current idea for testing is that i want the NANO to generate random numbers and send it back to the UNO.
[Slave-NANO Every](https://github.com/bridystone/SPI_Arduinos/blob/master/SPI_NANO_Every/src/main.cpp)
[Master-UNO](https://github.com/bridystone/SPI_Arduinos/blob/master/SPI_UNO/src/main.cpp)

This is now done with a lot of register setting and bit analyzing.

I'm just wondering if this is *not* planned in the implementation of SPI?
Is Arduino always planned as a MASTER?
now even AttachInterrupt and DetachInterrupt are hidden in private...

using Begin / BeginTransaction & Transfer did not really help.

Have i overlooked something?

Start with SPI_NANO_Every/src/main.cpp and SPI_UNO/src/main.cpp, then inspect the core entry points named Begin, BeginTransaction, and Transfer. Reproduce the linked master/slave experiment and determine the supported path for slave operation; done means the NANO can send generated values back to the UNO without the register-heavy workaround.

Written by the indexing model from the issue text.