Put the cashews in a bowl and cover with a couple of inches of water. Soak for 1 to 2 hours or overnight. (For a quick-soak method, cover with boiling water and soak for 30 to 60 minutes.) Drain and rinse. Transfer the cashews to a high-speed blender along with ½ cup (125 mL) of the water. Blend on high until super smooth and creamy in texture. Set the cashew cream aside.
In a large Dutch oven or stockpot, heat the oil over medium heat. Stir in the onion, garlic, and a couple pinches of salt, and sauté until the onion is softened, 4 to 6 minutes.
Stir in the carrots and celery, and cook for another few minutes or so. Stir in the cumin, thyme, and turmeric until combined.
Add the diced tomatoes with their juices, lentils, broth, and remaining water. Increase the heat to high and bring to a low boil. Reduce the heat to medium and simmer, uncovered, for 30 to 35 minutes, until the lentils are tender.
Stir in the cashew cream and chard. Add salt, pepper, and vinegar to taste. (The vinegar’s role is to add a little bit of brightness to the soup; add a bit at a time and keep tasting, as it can quickly overwhelm.) Cook for a couple of minutes over low-medium heat, until the chard is wilted, and then serve. This stew will keep in an airtight container in the fridge for up to 5 days, or you can freeze it for 1 to 2 months (always let it cool completely before storing). The stew will thicken after sitting in the fridge; you can thin it out with a bit of broth when you reheat it, if desired, or simply serve it thick with some crusty bread.