Preheat the oven to 400°F / 200°C. Line a baking tray with baking paper.
Put the butternut squash on the tray, drizzle avocado oil, sprinkle salt and toss with your hands to coat the squash in oil. Spread it over the tray and bake until tender and golden brown, about 20-25 minutes, stirring occasionally.
Warm about 3 tablespoons of oil in a medium skillet over medium heat. Add the mushrooms and garlic and saute for 5 - 7 minutes, stirring occasionally. Don't add salt , it would make the mushroom release their liquids. Remove from the heat.
Put the butternut squash into a large serving plate and add the mushrooms over. Sprinkle a little bit of salt.
Zest the lemon over and squeeze the juice over the squash and mushrooms. Drizzle the tahini butter over and sprinkle the walnuts. Garnish with the herb and serve.