Brown the Pork. Heat a little oil in a large, deep pot over medium-high heat. Add the pork shoulder and pork neck bones and cook until browned.
Build the Sinigang Broth. Add the tomatoes, onion, crushed garlic, water, tamarind and calamansi sinigang mixes, and fish sauce. Bring to a boil.
Simmer Until Tender. Cover, and simmer for approximately 45 minutes, or until the pork is tender.
Add the Vegetables. Add the eggplant, okra, string beans or green beans, and long green peppers. Cook until the vegetables are tender.
Add the Spinach. Add the spinach and cook just until wilted.
Taste and Adjust the Broth. Taste the broth. Add more fish sauce if needed. If it's too sour, add a little water at a time until it reaches your preferred level of sourness.
Serve and Enjoy. Serve hot with steamed white rice.