Determine direction of Slerp

How do we control the path that Slerp will take when the two vectors are pointing exactly the opposite directions?

Example below; how do we use Slerp and tell it to rotate along the green path from negative x to positive x, as opposed to blue?

Actually Slerp isn’t defined for an angle of 180°. It will result in an anomaly since the denominator will be “0”. So Unity’s Slerp implementation most likely picks a hardcoded direction if no direction can be determined. You can’t change the behaviour. If you want a specific direction you shouldn’t use Slerp.