Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Dice sweet potato into 1/2-inch pieces. Halve, peel, and slice onion into 1/2-inch-thick wedges. Trim and thinly slice scallions, separating whites from greens. Dice tomato. Zest and quarter lime.

Toss sweet potato on one side of a baking sheet with a drizzle of oil, salt, pepper, and half the Southwest Spice. Toss onion on opposite side with oil, salt, and pepper. Roast on top rack, tossing halfway through, until browned and tender, about 25 minutes.

Heat a drizzle of oil in a small pot over medium-high heat. Add scallion whites and cook until softened, 1-2 minutes. Add rice, 3/4 cup water (11/2 cups for 4 servings), and a large pinch of salt. Bring to a boil, then cover and reduce heat to low. Cook until tender, about 15 minutes. Keep covered until ready to serve.

Meanwhile, in a small bowl, combine sour cream, juice from 1 lime wedge, and a pinch of chipotle powder (start with 1/4 tsp and add more to taste). Stir in water, 1 tsp at a time, until mixture reaches a drizzling consistency. Season with salt and pepper.

Pat chicken dry with paper towels; season with salt, pepper, and remaining Southwest Spice. Heat a drizzle of oil in a medium pan over medium-high heat. Add chicken and cook until browned and cooked through, 4-6 minutes per side. Transfer to a cutting board. Let rest 2 minutes, then slice crosswise.

In a second small bowl, combine tomato, scallion greens, juice from 1 lime wedge, salt, and pepper. Fluff rice with a fork; stir in 1 TBSP butter (2 TBSP for 4 servings), lime zest (to taste), juice from 1 lime wedge, salt, and pepper. Divide rice between bowls. Top with veggies, chicken, and salsa. Drizzle with chipotle crema. Serve with remaining lime wedge on the side.