Preheat a large, heavy saucepan over medium heat for about 1 minute. Spray it twice with the vegetable oil.
Saute the onion for about 5 minutes, until translucent. Add 4 cups of the corn and saute' for 4 to 5 minutes, until it softens a bit. Add 2 cups of the chicken stock and cook until the corn can be mashed easily with a fork, about 20 minutes.
Transfer the contents of the pan to a blender and puree until smooth.
Return the puree to the saucepan over medium-low heat. Add the bell pepper, rosemary, thyme, black pepper, cayenne pepper, and the remaining 1 cup chicken stock and 2 cups corn.
Stir and cook for about 10 minutes more, until the chowder is thick and creamy.
Garnish with the chopped basil.