Preheat a grill to low heat.
Rub the meat all over with 1/4 cup of the olive oil and season liberally on all sides with salt and pepper. Place the meat on the grill, fatty side down first, and cook, turning occasionally, until meat is nicely browned on all sides and an instant-read thermometer inserted into the center registers 120 degrees F, 1 hour and 20 minutes and maybe longer depending on your grill. Remove from the grill and set aside to rest for at least 15 minutes before serving.
In a mortar and pestle or small bowl mash the salt and pepper, to taste, garlic, oregano and rosemary until combined and then mash in the remaining olive oil until a paste forms.
When ready to serve, cut the meat away from the bone so that it is in 2 large pieces. Slice the pieces into thin slices and serve drizzled with the herbed garlic oil and sauteed spinach.