Preheat oven to 425°. In a large bowl, toss sweet potatoes, oil, apples, syrup, lemon zest and juice, chopped thyme, 1 1/2 tsp. kosher salt, and pepper until coated.
Spread mixture in a single layer in 2 large, oiled, rimmed baking pans. Roast until sweet potatoes are browned and tender when pierced, 40 minutes, turning chunks every 15 minutes. Transfer to a platter, add more salt to taste, and garnish with thyme sprigs.
Make ahead: Up to 1 day; chill. Reheat, drizzled with a little olive oil, in a 425° oven in 2 baking pans, until hot, 20 minutes.
Note: Nutritional analysis is per serving.