Due to dodges, parries and their decent dps, one handed weapons can safely be used without shields and some people prefer not to use bows. This means the left hand is empty for most of the time, which opens the opportunity for off-hand magic.
The weapon could be something discreet, like a small orb, or ring that fits in or on the off-hand. It would rely on focus to cast and focus would be built up the usual way using the main hand attacks and parries.
I was just about to make a post like this, but you beat me to it. Ever since they removed torches as an option, which I thought still existed. Most of my builds in Soulslike games like this and Elden Ring, I always make a One-handed weapons with an off-hand magic option. The option of lacking a reliable parry option/shield with more offensive magic would’ve been great with torches, but the Breach update removed it and I am bummed out about it.