Preheat oven to 300° F.
In a small bowl, combine salt, black pepper, ground cinnamon, garlic powder, and paprika. Set aside.
Heat oil in a large skillet on the stovetop over medium heat.
Rub the spice mixture over the top, bottom, and sides of the beef, pressing to ensure it sticks.
Place the prepared corned beef in the preheated skillet and sear it for 1-2 minutes.
Flip it over and sear the other side for an additional 1-2 minutes. Remove the skillet from the heat and set it aside.
Add beef broth and maple syrup to a large roasting pan and whisk to combine.
Lay the seared corned beef into the pan and arrange the onion slices, orange slices, and fresh rosemary sprigs on top and around the beef.
Cover the roasting pan with aluminum foil and bake in preheated oven for at least 3 hours, or until the corned beef reaches an internal temperature of 180-205° F.
Remove the corned beef from the oven and transfer it to a cutting board. Let it rest for 10–15 minutes, then slice against the grain with a serrated knife.
Move the slices to a serving platter, spooning the pan juices over the top. Serve as desired.