Preheat oven to 400°F (200°C). Line a baking pan with parchment paper or foil and drizzle 1 tablespoon of olive oil over the surface.
Scrub the sweet potatoes and slice each in half lengthwise, leaving the skin on. Place cut-side down on the prepared pan and rub into the oil to coat.
Cover with foil and roast for 20 minutes. Remove the foil and roast for another 25–30 minutes, or until soft and golden underneath.
In a small bowl or measuring cup, whisk together tahini, maple syrup, lemon juice, Dijon mustard, salt, and pepper. The mixture will thicken - gradually add water, a tablespoon at a time, until smooth and pourable. Set aside.
When the sweet potatoes are nearly done, heat the remaining 1 tablespoon olive oil in a skillet over medium heat.
Add the diced onion and cook for 5–7 minutes, until soft and starting to brown. Add the chickpeas, minced garlic, harissa paste, cumin, and salt. Cook for another 5–7 minutes, stirring frequently, until the chickpeas are heated through and slightly crisped.
Flip the roasted sweet potatoes cut-side up. Spoon the harissa chickpeas over each half. Drizzle with tahini sauce, sprinkle with chopped cilantro, and serve with baby spinach if desired.