Preheat the oven to 400°F (200°C).
Soak the cashews in hot water for 5 minutes, then drain; alternatively simmer in boiling water for 5 minutes. Set aside.
Peel and finely chop the onion. Heat a little oil in a non-stick frying pan over medium heat and cook the onion until translucent and glossy, about 5 minutes.
Add the 2 chopped garlic cloves to the pan and cook 1–2 minutes until fragrant.
Stir in the chopped tomatoes, tomato puree, paprika, basil, oregano, and salt and pepper to taste. Simmer for about 5 minutes to meld flavors.
Add the cooked or canned brown lentils to the pan and cook 1–2 minutes to heat through; remove from heat — this is your lentil bolognese ragu.
Make the white cashew sauce: place the drained soaked cashews, 1 whole garlic clove, dairy-free milk, nutritional yeast, nutmeg (if using), and salt and pepper into a blender or food processor and blend until very smooth and creamy.
Spoon a thin layer of lentil ragu into the bottom of the oven-safe dish to prevent sticking.
Assemble the lasagna: layer pasta sheets, lentil ragu, and white cashew sauce. Repeat layers until you have used about 6 layers of pasta and finished with a top layer of white sauce. Sprinkle the dairy-free grated cheese or vegan parmesan (or extra nutritional yeast) on top if using.
Bake the lasagna in the preheated oven for 30 minutes, then remove and let sit 5–10 minutes before slicing and serving.
