Preheat the oven to 425ºF. Line a large rimmed baking sheet with a silicone mat or use a nonstick sheet.
Roast the sweet potato: Transfer the sweet potato to the prepared baking sheet. Add the thyme, cinnamon, nutmeg, salt, and pepper. Drizzle with olive oil and toss to coat evenly with the oil and seasoning. Roast for about 20 minutes, or until fork tender, tossing halfway.
Cook the quinoa: Transfer the quinoa with 2 cups water to a medium saucepan. Bring to a boil before reducing the heat. Simmer covered for 15 minutes. Remove from the heat and set aside.
Cook the onions: Meanwhile, cook the onions. Heat 1 tablespoon olive oil in a large nonstick skillet over medium-high heat. (Or add 1 tablespoon coconut aminos with the onions.) Cook the onions for about 20 minutes, or until golden brown, stirring occasionally. Drizzle with more coconut aminos and cook for 1 minute more (optional).
Prepare the dressing: Meanwhile, to a medium jar, add the tahini, lemon juice, Dijon, maple syrup, and ⅓ cup water. Grate the garlic into the jar using a microplane or fine grater. Place the lid on the jar and shake vigorously until the ingredients are well combined. Add salt and pepper.
Assemble for serving: Use a fork to fluff the quinoa. Transfer the quinoa to a large bowl. Mix in the arugula. Add the sweet potato, onions, and cashews and mix to combine. Drizzle with the desired amount of tahini dressing for serving.