1. Preheat the oven to 425°. In a medium bowl, toss together the olive oil, garlic, ½ teaspoon of the oregano, onion, tomatoes and tomatillos. Arrange on a baking sheet, season with salt and place in the oven. Roast until the tomatoes and tomatillos are soft, about 20 minutes. Remove from the oven and set aside to cool.

2. Lower the oven temperature to 250°. Season the ribs with salt, pepper and the remaining teaspoon oregano. Set a grill pan over high heat and sear the ribs until slightly browned, about 4 minutes on each side. Place the ribs on an aluminum-foil-lined rimmed baking sheet and cook for 1 hour and 30 minutes.

3. Meanwhile, make the mole: Bring the water to a boil. Place the chiles in a large bowl and pour boiling water over them until they are submerged. Top the bowl with a wet paper towel and set aside until the chiles are rehydrated, about 30 minutes. Drain the chiles.

4. In a food processor or blender, purée the cooled tomato-tomatillo mixture with the rehydrated chiles, toasted pepitas and chicken broth. Strain the mixture through a fine-mesh sieve and into a medium saucepan. Cook over medium heat for 10 minutes, then stir in the brown sugar, chocolate, cinnamon and allspice. Season with salt and black pepper and simmer for 10 minutes. Cover to keep warm.

5. Brush some of the mole over the ribs and return the ribs to the oven until tender, an additional 1 to 1½ hours. Brush with more sauce and serve hot with additional sauce on the side.