Directions

1. In a large pot, add the pork shoulder, whole onion, head of garlic, whole green chili and salt to taste. Cover with water. Heat over high heat until boiling, then lower the heat to medium to simmer, about 1 hour until pork is tender.  Remove pork, setting it aside to cool. Reserve the broth. Remove any skin and bones and shred.
2. Remove the onion, garlic, and chili from broth.  Strain broth in a blender jar to remove excess residue. Add 2 small cans of tomato puree and 1 chipotle chile, salt, pepper and oregano, and blend well until smooth.
3. In a large skillet over medium high heat, heat oil.  Add onion slices and garlic and cook until softened but not browned.  Add tomato mixture and heat to boil; add pork and cover skillet. Turn down heat and simmer for 5-10 minutes.
4. Serve pork on tostado and top with avocado slices.