Step 1Heat the oven to 300 degrees.
Step 2In a large heavy Dutch oven heat the olive oil over medium-high heat. Sprinkle the ribs with one-half teaspoon salt and one-half teaspoon freshly ground pepper. Brown the ribs on all sides, in batches if necessary, about 15 minutes. Remove the ribs from pan and place in a bowl. Turn off the heat and carefully pour out half of the fat.
Step 3Turn the heat to medium. Add the onions, garlic, avocado leaves and chiles. Break up the achiote paste and sprinkle it over the onion. Cook for 5 to 10 minutes, until onion is very soft. Add the tomatoes, coriander, fennel, mustard, cumin and pumpkin seeds and the cinnamon stick. Continue to cook, stirring occasionally for 10 minutes.
Step 4Add the red wine and reduce to almost dry, about 5 minutes. Add the ribs, water, and chicken and veal stock. Stir and cover with a tight-fitting lid.
Step 5Braise the ribs in the oven for 3 hours. Remove the pan from the oven, carefully remove the lid and check texture of the ribs with tongs. If the ribs are soft to the touch they are finished; if they are still a little tough, replace the lid and cook for another 30 minutes.
Step 6Place the ribs in a serving casserole and cover to keep warm. Pour the braising liquid into a bowl and spoon off the layer of fat. Pour into a blender in batches and carefully puree the braising liquid. (Be careful when blending hot liquids; place a kitchen towel over the top of the blender to catch any spatters.) Add a teaspoon of salt and one-fourth teaspoon pepper to sauce or more to taste. Pour sauce over the ribs. Serve with mashed potatoes or creamy polenta.