Cube the pork into 1" or so cubes and brown in the oil.
Add the onions and cook a few minutes, stirring.
Add the chilies, tomatoes (undrained), garlic, and water.
Bring to a simmer, cover partially and cook 2-3 hours. Add more water if necessary and stir periodically.
When you're ready to serve, mix the cornstarch and water, then stir into the chili and simmer a few minutes to thicken.