Cover the cashews with boiling water and allow to soak for 1 hour. When done soaking, drain well and rinse.
Preheat the oven to 400°F/205°C. Line two large sheet pans with parchment paper (optional, but for easy cleanup).
Prepare the vegetables. Cut the cauliflower into florets, and chop the peeled sweet potato into roughly equal size as the florets. Chop the carrots into slightly smaller pieces than the sweet potato (they take longer to cook). Toss the vegetables with the oil and generously season with salt and pepper. If the veggies seem dry, add a bit more oil as needed. Spread the veggies out in a single layer on the prepared sheet pans. Roast in the oven until they are browned in some spots and soft, tossing once during cooking, about 25 to 30 minutes. Reduce the oven temperature to 350°F/175°C (or turn off until ready to assemble the bake).
While the vegetables are roasting, cook the lentils. Heat a deep sauté pan or Dutch oven over medium heat and add the 1 1/2 tablespoons olive oil. Add the diced sweet onion and cook until lightly browned, 5 to 7 minutes. Add the garlic and cook, stirring frequently, until fragrant, about 2 minutes.
Deglaze the pan with the vegetable broth or water, and then add the French green lentils and thyme sprigs. Bring to a boil, and then reduce to a rapid simmer. Simmer until most of the liquid is absorbed and the lentils are tender but still al dente, 25 to 30 minutes. Remove the thyme sprigs and add the tahini, miso paste, and balsamic vinegar. Stir well to combine and taste for seasonings, adding salt or pepper as desired.
While the vegetables are roasting and the lentils are cooking, make the cashew cream. To a high-powered blender or food processor, add the soaked and drained cashews, water, garlic, nutritional yeast, lemon juice, 1/2 teaspoon sea salt and black pepper to taste. Blend until smooth and creamy. If you need a bit more water to blend the cashews, add 1-2 more tablespoons. You can also prepare the cashew cream a few days in advance, if desired.
Assemble the bake: If the oven is off, heat it 350°F/175°C. Layer the roasted vegetables on the bottom of a 13×9-inch baking pan or 3-liter casserole (or in two smaller baking pans). Top with the cooked lentils and then pour the cashew cream on top, smoothing it out over the entire surface. Bake for 20 minutes, until warmed through. If desired, turn on the oven broiler and broil for a minute or two until the cashew cream is browned in some spots.
Store leftovers in an airtight container in the fridge for up to 3-4 days.