Place pork, onion and garlic in 2-quart saucepot; cover completely with water and bring to a rolling boil. Reduce heat and simmer for 1-1/2 hours or until meat is tender. Drain meat (discard garlic and onion). When cool enough to handle, remove bones, shred pork and store in refrigerator until ready to use.
Cut tomatoes in half and discard stems and seeds. Place in shallow pan and brush with oil. Roast in 375° F oven 20 to 30 minutes or until tomato skins slip off easily. Discard skins and chop tomatoes.
Heat oil in medium skillet. Cook and stir onion and garlic over medium-high heat 3 minutes or until tender. Stir in tomatoes, cover, reduce heat and simmer 5 minutes or until tomatoes are very saucy. Add remaining sauce ingredients and shredded pork and simmer 2 to 3 more minutes or until mixture is well blended and heated through.
Serve hot meat mixture on tostaditas with crema and avocado.