While the squash is roasting, place cashews in boiling water (use a small saucepan) and simmer for 15+ minutes.
Boil water for pasta in a large pot, adding 2-3 tablespoons of salt. If you've made butternut squash, your oven should still be at (or close to) 350 F.
Slice onions, dividing into halves. Then slice mushrooms. Mince garlic and set aside.
Place half of the sliced onions in a pan on medium heat with a splash of grapeseed oil. Once soft and slightly translucent, add garlic and thyme. Cook for 1-2 more minutes, reducing heat to low, then remove from heat.
When butternut squash is done, place all sauce ingredients into the blender. Make sure everything is WARM or HOT so the sauce fully blends. Start blending on low, and work up to higher speeds, blending for at least 2 minutes in a high-power blender (like a Vitamix). Reminder: use extra caution when blending hot liquids. Add the rest of the onion to a pan on medium heat with a splash of grapeseed oil. Cook until onions are translucent. Then add mushrooms and a pinch of salt. Cook mushrooms until they release juices, then add soy sauce and spinach. Let spinach wilt (1-2 mins), then remove from heat.
Cook lasagna noodles for half the recommended time (usually 4-6 minutes).
Layer the lasagna: Start with a small layer of cheese sauce. Then noodles, sauce, mushrooms & onions, and mozzarella. If you have extra squash cubes, layer those as well! Repeat until top layer. On top, add sauce & the rest of the mozzarella.
Cover with foil and bake for 20-30 minutes. To keep warm, lower heat to 200 F for up to 3 hours. Enjoy!