How to Make It
In a small saucepan, cover the annatto seeds with water, bring to a boil and simmer over moderate heat for 3 minutes. Remove from the heat and let stand for 2 hours. Drain the seeds and pat dry with paper towels. Transfer to a spice grinder and grind to a paste.
Preheat the oven to 500°. Put the 6 whole garlic cloves in a pie plate and roast on the top rack of the oven for about 10 minutes, or until blackened on top. In a food processor, puree the roasted garlic with the halved garlic cloves and the onion. Add the annatto paste, oregano, pepper, grapefruit and orange juices and oil and process until blended.
Rub the spareribs with 2 tablespoons of kosher salt. Set 1 of the racks in a large roasting pan and coat with one-third of the annatto marinade. Repeat with the remaining ribs and marinade, stacking the racks on top of each other. Cover with plastic wrap and refrigerate for at least 4 hours or overnight.
Preheat the oven to 350°. Line each of 3 large rimmed baking sheets with a banana leaf. Set a rack of ribs on each leaf, meaty side up. Arrange 6 bay leaves and one-third of the red onion slices on each rack and cover with a second banana leaf. Cover each pan tightly with foil and bake for about 2 1/2 hours, or until the ribs are very tender; shift the pans halfway through cooking. Remove the spareribs from the oven and let cool, covered, for 15 minutes. Meanwhile, wrap the tortillas in foil and warm in the oven for about 10 minutes.
Preheat the broiler. Uncover the ribs and discard the banana leaves, onions and bay leaves. Broil the ribs, 1 rack at a time, 8 inches from the heat, for about 4 minutes, until crispy; baste occasionally with the pan juices.
Cut the racks into ribs and serve with the warm tortillas and bowls of Pickled Red Onions and Yucatán Table Sauce.
Banana leaves are available in the freezer section of Latin and Asian markets and in the fresh produce section of some supermarkets.