Combine the oil and flour in an 8-quart pot over medium heat and stir with a wooden spoon until smooth. Cook the mixture, stirring constantly, to make a blond roux, about 8 minutes.
Add the onions, celery, salt, cayenne, bay leaves, garlic, stock, and ham hocks. Bring the mixture to a boil, reduce the heat to medium-low, and simmer, uncovered, until the hocks are very tender, about 2 hours.
Add the greens, by the handful, until all of them are combined in the mixture. They will wilt. Add the water. Simmer until the greens are very tender and the mixture is thick, about 45 minutes.
Remove the bay leaves and serve warm.
Yield: 8 to 10 servings