For the dry rub and ribs: Combine the paprika, cumin, salt and pepper in a small bowl. Pat the spice mixture all over the ribs. Cover with plastic wrap and refrigerate 2 to 4 hours or overnight.

Preheat the oven to 300 degrees F, or 250 degrees F with fan on in a convection oven.

Place ribs in a single layer in a baking pan and pour in water to a depth of about 1/2-inch. Bake, covered with foil, for 45 to 60 minutes in the oven (or on a grill with the lid closed). Uncover and bake until the meat is tender and beginning to pull away from the bones, an additional 45 to 60 minutes. Remove the ribs from the pan and reserve the pan juices to add to tomatillo glaze.

Meanwhile, make the tomatillo glaze: In a large saucepan over medium-high heat, saute the onions in the duck fat until golden. Add the garlic, tomatillos and serrano chiles and cook, stirring frequently, until the mixture thickens, 30 to 45 minutes. Add the reserved rib pan juices and cilantro and cook an additional 10 minutes. Puree with an immersion blender and add the maple syrup. Cook over low heat for 10 minutes, stirring constantly and being careful not to burn. Season with salt and pepper.

Turn the oven heat up to 450 degrees F, or preheat the grill. If finishing the ribs in the oven, brush generously with the glaze and bake another 10 minutes per side, basting with the glaze every 2 to 3 minutes. To grill, generously glaze the ribs and grill 5 minutes per side, frequently brushing with additional glaze. Cut the ribs apart and serve hot with extra glaze on the side.