Heat a large soup pot or Dutch oven over medium heat. Once the pot is hot, pour in the olive oil and swirl it around. Add the onions and stir. Sauté the onions until translucent and very soft, about 5 minutes. If the onions start to brown on the edges, lower the heat.
Add the rosemary and garlic to the pot and stir until very fragrant, about 30 seconds.
Add all of the sliced mushrooms to the pot. Give them a quick stir and then let them sit for a full minute. Once the minute is up, stir them up and then let them sit again for another full minute. Stir them up after that and season them with pepper. Let them sit for another full minute. Once they start glistening, season the mushrooms with salt. Keep stirring and sautéing the mushrooms until they are tender, about 3 additional minutes.
Add the balsamic vinegar to the pot and stir. Let some of the strong vinegar aroma evaporate a bit. The mushrooms should be pretty juicy at this point!
Remove ⅓ of the sautéed mushroom mixture and place it in an upright blender. Also in the blender, add the drained cashews, miso, nutritional yeast, and 2 cups of the vegetable stock. Blend this mixture on high until completely smooth and creamy, about 30 seconds.
Pour the remaining vegetable stock into the pot. Add the cooked farro to the pot as well.
Pour the blended mushroom and cashew cream into the pot and stir. bring the stew to a boil and let it simmer, uncovered, for about 10 minutes—just to let the stew thicken slightly and to let the flavours meld. Then, add the kale to the pot and stir. Keep simmering the stew until the kale is wilted, about 3-4 minutes.
Check the stew for seasoning and adjust if necessary (more salt, balsamic vinegar, pepper etc). Serve the stew hot!