Add pork, onion, bay leaf, garlic, oregano, cumin and salt and pepper to slow cooker. Cover the pork with water and cook on LOW for 8 hours or overnight.

Drain the pork, reserving the broth. Shred the meat.

Remove the stems and seeds from the dried chiles while rinsing them under cold water. Place chiles in a saucepan with 2 cups of broth from the cooked pork. Simmer, uncovered, for 20 minutes, then remove from heat.

Transfer the chiles and broth to a blender. Add oregano, cumin, chili powder, onion powder, garlic and blend until smooth. Set aside.

In the same saucepan you used to cook the chiles, add oil and cook over medium heat. Add flour and salt and stir for 1 minute.

Strain the chili sauce from the blender and add it to the saucepan. Cook, stirring occasionally, for 10 minutes, then add the sauce to the shredded pork.