How to receive bytes from SPI with interrupts using HAL on STM32?
Feeling stupid, but found the answer. Just want to mention it for others:

The result is NOT inside the argument hsp (.pRxBuffPtr) of the interrupt handler

void HAL_SPI_RxCpltCallback(SPI_HandleTypeDef *hspi)


but in the variable _spi_data provided by the receive command:

HAL_SPI_Receive_IT(&amp;hspi1, _spi_data, 10);