Make the cashew cream by blitzing the raw cashews with ¼ cup water until very smooth. Set aside.
Heat the oil and add the onions or shallots. Season with salt and ground black pepper to taste. Cook, stirring frequently, over medium heat until the onions caramelize and turn golden-brown.
Add the mushrooms with the apple cider vinegar and cook, stirring frequently, until the moisture has evaporated.
Add the flour and cook, stirring, 2-3 minutes, until the flour is completely absorbed by the veggies.
Add two cups of the stock, whisking as you add it, until you have a smooth sauce. Add the remaining stock, continuing to whisk.
Stir, add the soy sauce and sage, and continue to cook for another 15-20 minutes over high heat or until the gravy is reduced by almost half and is thick and creamy. Stir in the cashew cream.
Turn off the heat and, at this point, you can strain the sauce for serving. Or, if you'd rather, serve it as is. The bits of mushroom and onion in the gravy are delicious.