Collard and Mustard Greens in Ham Hock Gravy

Ingredients

1/2 cup vegetable oil
1/2 cup bleached all-purpose flour
2 cups thinly sliced yellow onions
1/2 cup chopped celery
1/2 teaspoon salt
1/4 teaspoon cayenne
4 bay leaves
2 tablespoons chopped garlic
8 cups Chicken Stock [3]
3 pounds ham hocks (about 4 medium-size hocks)
2 bunches (about 2 1/4 pounds) each of collards and mustard greens, thoroughly washed, picked over for blemished leaves, and tough stems removed
1 cup water

Combine the oil and flour in an 12-quart pot over medium heat and stir with a wooden spoon until smooth. Cook the mixture, stirring constantly, to make 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, about 45 minutes.
Remove the bay leaves and serve warm.