Preheat your oven to 375°F.
In a small bowl, whisk together the apple cider, maple syrup, melted butter, salt, black pepper, dried thyme, garlic powder, onion powder, paprika, and cinnamon.
Rinse the Cornish hens under cold water and pat them dry with paper towels. Place them in a roasting pan, breast side up.
Pour half of the cider-maple mixture over the hens, making sure to coat them evenly. Reserve the other half for basting.
Stuff the cavity of each hen with the quartered apple and onion.
Roast the hens in the preheated oven for about 1 hour and 15 minutes, or until the internal temperature reaches 165°F. Baste the hens with the reserved cider-maple mixture every 30 minutes.
Once cooked, remove the hens from the oven and let them rest for 10 minutes before carving. Garnish with fresh thyme sprigs if desired.