Classic Green Chile Sauce (Chile Verde)

This can be used in any recipe needing a green chile sauce.

8 chopped green New Mexican chiles, roasted,
    peeled, stems and seeds removed (optional)
1 medium onion, chopped
2 cloves garlic, chopped
2 tablespoons vegetable oil
1 medium tomato, peeled and chopped
1/2 teaspoon ground cumin (optional)
2 cups water

Saut? onion and garlic in oil until softened. Add remaining ingredients, bring to a boil, reduce heat, and simmer for 30 minutes.