New Mexican Green Chile Stew

2 pounds lean pork, cubed
2 tablespoons vegetable oil
1 large onion, chopped
2 cloves garlic, minced
6 to 8 New Mexican green chiles, roasted and
    peeled, seeds and stems removed, chopped
1 large potato, peeled and diced (optional)
2 tomatoes, peeled and chopped
3 cups water

Brown pork in the oil. Add onion and garlic; saut