Toss together the sweet potatoes, onion, 1 tablespoon oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper on a rimmed baking sheet. Roast 20–25 minutes or until tender.
Meanwhile, heat remaining 1 tablespoon oil in a skillet over medium-high heat. Season chicken with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper, and cook for 5–6 minutes per side or until chicken is golden and cooked through.
Toss the sweet potato–onion mixture with spinach and fresh lime juice. Serve warm with chicken.
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