Houston's Canadian Cheese Soup

Source: Houston's Restaurant

1/2 cup margarine
1 cup carrots, 1/8-inch cubes
1/2 cup celery, 1/8-inch cubes
3 cups chicken broth
3 cups half-and-half
2 pounds Velveeta cheese, cut into small cubes
1 tablespoon fresh parsley, chopped fine
Diced tomatoes or diced jalape