Preheat oven to 400ºF. Place whole squash on a baking tray or in a casserole dish. Roast for about 35 to 45 minutes, or until you can easily pierce the squash with a sharp knife. Remove from the oven. When cool enough to handle, cut the squash in half width-wise, which allows for longer strands. Use a fork to pull out the seeds and discard (or clean them and roast with oil, salt, and pepper). Use a fork to gently scrape strands of squash into a large bowl or serving dish. This can be done several days in advance and stored covered in the refrigerator. While the squash is roasting, line a baking tray with parchment paper. Put chickpeas and pumpkin seeds in a medium bowl. Drizzle with 1 tablespoon of the oil. Sprinkle with cumin, garlic, onion, smoked paprika, and 1 teaspoon of the salt. Toss to coat. Scoop out onto the prepared baking tray and arrange in a single layer. Roast in the oven for about 15 to 20 minutes, or until the chickpeas are golden brown. Remove from the oven and set aside. To serve, flavor spaghetti squash by tossing with remaining 1/2 teaspoon salt, 1 tablespoon of olive oil, and lemon juice. Transfer chickpeas and pepitas to bowl with squash. Add parsley and gently toss everything together. Drizzle with tahini dressing and serve immediately at room temperature or refrigerate and serve cold. This dish can also be served hot. To make the tahini dressing, whisk together all the ingredients in a small bowl and stir until smooth. Add more water to thin, if necessary.