If SPI settings are set to SPI_MODE3 clock signal should idle at HIGH but before doing:

Code:
SPI.beginTransaction(LTC6811_SPI_config);
and after doing SPI.begin(); being

Code:
LTC6811_SPI_config = SPISettings LTC6811_SPI_config(SPISettings(500000, MSBFIRST, SPI_MODE3));

the clock is idling LOW.

The problem of this behaviour is that the first message is ignored by the slave. After doing
Code:
SPI.beginTransaction(LTC6811_SPI_config);
for the first time the clock will start idling at HIGH as expected.

This bug is present at least with the Teensy 3.2 using VSCode and Platformio but I think this may be related to the Arduino SPI library itself since the function void SPIClass::begin() in the Arduino SPI library only does pinMode(SCK, OUTPUT); which by default sets the SCK pin LOW. This function should take into consideration the SPI_MODE that the user whats to operate in.