In a large pot over medium heat, warm 1 tablespoon oil. If your sausage is raw, add it to the skillet and let cook, undisturbed, for 5 minutes or until browned, breaking it up into chunks. Give it a stir and let cook for another minute or so until cooked through. Transfer to a plate.
Add the remaining oil to the pot along with the onion, carrot, celery, and ½ teaspoon salt. Cook for 5 minutes or until softened, stirring occasionally.
Add the garlic, thyme, rosemary, and red pepper flakes (if using) and stir for 30 seconds. Stir in the water, lentils, soy sauce, sausage, and remaining ½ teaspoon salt.
Cover and increase the heat to high to bring to a boil. Give it a stir, cover, and reduce the heat to medium-low to maintain a rapid simmer for 15 minutes, or until the lentils are cooked through.
In a high-speed blender, add the drained cashews and about 1 cup of the liquid from the soup (it’s okay if some lentils/veg get in there but leave the sausage out). Blend on high for about 1 minute until very smooth (make sure you vent the lid so steam can escape), then pour back into the pot. Stir in the kale and lemon.
Serve topped with cheese (if using), and black pepper. You could also top with some fresh herbs if you have any.