Preheat the oven to 425°F (220°C). Line two large baking sheets with parchment. Scrub and dry the yams, then slice in half lengthwise. Rub 1 tablespoon olive oil over the cut sides and place cut-side down on one sheet. Roast 20 minutes.
Meanwhile, warm the remaining 2 tablespoons olive oil in a small skillet over low heat. Stir in smoked paprika, cumin, coriander, and sumac for 30–45 seconds until fragrant (do not scorch). Pour the spiced oil over the well-dried chickpeas and toss to coat.
Remove yams, flip cut-side up. Spread chickpeas in a single layer on the second sheet. Return both sheets to the oven and roast 15–20 minutes, until yams are fork-tender and chickpeas are golden; shake/turn chickpeas once halfway. For extra crispness, leave chickpeas in 5–10 minutes more after yams are done.
In a small bowl, whisk tahini, cold water, lemon juice, grated garlic, maple syrup, and salt until smooth and pourable (add cold water 1 tablespoon at a time if needed).
In another bowl, combine tomatoes, cucumber, parsley, and red onion. Toss with lemon juice and olive oil.
For 6 servings: place two roasted yam halves (1 whole yam) on each plate. Fluff flesh with a fork, top with spiced chickpeas and herb salad, and finish with a generous drizzle of lemon-tahini sauce. Serve immediately.