Preheat your oven to 375°F.
Slice the eggplant into ¼-inch rounds, lightly brush with olive oil and season with salt and pepper; grill or roast them until tender, about 10 minutes per side.
In a skillet over medium heat, warm 1 tsp olive oil, add the chopped onion and minced garlic, and sauté until fragrant and softened.
Add the lean ground beef to the skillet, break it apart, and cook until browned; season lightly with salt and pepper.
Stir in the tomato sauce with the beef, and allow the mixture to simmer for 5 minutes to meld flavors.
For the cashew bechamel, combine raw cashews, unsweetened almond milk, and nutritional yeast in a blender; blend until smooth and creamy. Adjust seasoning with salt and pepper if needed.
In a casserole dish, layer the grilled eggplant slices, then evenly spread the beef and tomato sauce mixture, and drizzle a layer of the cashew bechamel sauce on top. Repeat layers if desired.
Bake the layered casserole in the preheated oven for 15-20 minutes until heated through and flavors have blended.
Remove from oven, let cool slightly, and serve warm.