Put the chicken and onion in a large bowl and sprinkle with the taco seasoning. Drizzle with the oil and toss well to coat. Cover with plastic and refrigerate for at least 2 hours and up to overnight.
Preheat the oven to 425 degrees F. Line a rimmed baking sheet with 2 layers of foil.
Arrange the chicken and onions on the prepared baking sheet in a single layer, skin-side up. Roast until the skin is crisp and the chicken is just cooked through or an instant-read thermometer inserted into the thickest part of the breast registers 160 degrees F, about 45 minutes.
While the chicken cooks, stir the crema together with the lime juice and zest and cilantro. Season with salt.
Arrange the chicken and onions on a serving platter and garnish with the cilantro sprigs. Serve with the lime crema on the side.