For the Cornish game hen: Preheat the oven to 400 degrees F. Set a wire rack over a baking sheet.
Toast the coriander and cumin seeds in a small skillet over medium heat until fragrant, a few minutes. Grind in a spice grinder. Combine half of the ground spices, the garlic powder, 1 teaspoon salt and 1/2 teaspoon pepper in a small bowl. Reserve the remaining ground spices.
Pat the hen dry with paper towels and season all over with the spice mixture. Drizzle with olive oil and place, skin-side up, on the prepared baking sheet. Roast until cooked through, 35 to 45 minutes. Cut the hen in half.
For the green mojo: Combine the olive oil, lime juice, cilantro, garlic, serrano and the reserved ground cumin and coriander in a high-powered blender. Blend until bright green and smooth. Season with salt and pepper.
Pour the sauce over the hen and serve with Mashed Citrus Sweet Potatoes.
Mashed Citrus Sweet Potatoes:
Place the sweet potatoes in a medium saucepan and cover by 1 inch with water. Bring to a boil over high heat and season with salt. Turn down the heat and simmer until the potatoes are cooked through, 10 to 15 minutes. Strain the potatoes, then add them back to the pot to dry. Mash the potatoes in the pan. Add the chicken stock, honey, thyme, lemon zest and juice and orange zest and juice. Add more chicken stock if needed. Season with salt and pepper.