1. Put the pork in a heavy Dutch oven large enough to fit it comfortably. Pour in enough cold water to cover the meat by 2 inches. Add the salt and bay leaf. Bring the liquid to a boil and simmer for 1 hour. Skim off the foam from the surface as necessary.
2. Meanwhile, remove the stems from the chilies and tap out the seeds. Heat the olive oil in a medium skillet over medium heat. Add half of the chilies and toast them, turning with tongs until they start to change color and crisp up a bit, about 4 minutes. Heat them gently so they don’t burn. Lift them out and place in a bowl. Repeat with the remaining chilies. Pour enough boiling water over the toasted chilies to cover them. Soak until completely softened, about 20 minutes. Drain well.
3. While the chilies are soaking, wipe out the skillet with paper towels. Place the onion and tomatoes, cut sides down, in the skillet. Cook, turning the vegetables as often as necessary, until the tomatoes are blackened on all sides and the onions are blackened on both flat sides.
4. After the pork has been cooking for 1 hour, remove 2 cups of the cooking liquid and pour it into a blender jar. Add the onions and tomatoes, and purée until smooth. Add the chilies and blend until smooth. Remove another 2 cups of the cooking liquid and set aside. Drain the pork, discarding the remaining liquid, and wipe out the pot.
5. Set the pot over medium-low heat and add the canola oil. Stir in the flour and cook, stirring, for 3 to 4 minutes. Pour the chili sauce into the pot slowly. As it comes to a boil, it will thicken. Stir well,especially in the corners, to prevent the sauce from sticking and scorching as it thickens.
6. Add the pork and return the liquid to a simmer, cover the pot, and cook until tender, about 1 hour. While the pork is cooking, there should be enough sauce to keep it moistened. If not, add the reserved pork cooking liquid as needed.