Preheat the oven to 450ºF, line a rimmed baking sheet with foil, and grease it with a bit of oil. Set aside.
Season the pork tenderloin with salt and pepper, rub with the harissa, tomato, and coconut aminos. Place the pork on the baking sheet. Toss the kabocha and red onion with the avocado oil. Spread them onto the baking pan around the pork. Roast for 20-25 minutes, or until the pork registers 145ºF. Transfer pork to a cutting board and tent with foil to rest at least for at least 5 minutes. Return the pan to the oven and continue roasting the squash and onions for another 10-15 minutes, or until they take on some golden color and are cooked through.
While the pork and squash cooks, prepare the tahini sauce by stirring together the tahini, water, lemon, and garlic. Add more water as needed to make the sauce a drizzling consistency.
Once roasted, divide the vegetables between two serving plates and drizzle with sauce. Sprinkle with za’atar and parsley. Slice the pork divide between the two plates and serve.