Combine the oil and flour in a 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 to the roux. Season with salt and cayenne. Cook the onions for about 4 to 6 minutes, or until the onions are soft. Add the bay leaves and garlic and continue to cook for 2 minutes. Add the stock and ham hocks. Bring the liquid to a boil and reduce to a simmer, stirring occasionally. Simmer the liquid for about 2 hours, or until the hocks are very tender. 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.