Put water and salt in a large dutch oven and bring to a boil on high heat. Reduce to medium-high and gradually add greens, onions and garlic. When all the greens have been added and the water returns to a boil, reduce the heat to a a simmer. Cover and cook for 30 minutes, stirring occasionally.
When done, drain well, reserving the liquid. Depending on the size of your food processor, you may need to work in batches to pulse the greens/onions until pureéd.
Make a roux in same dutch oven: heat oil over medium heat and add flour, continually stirring with a wooden spoon or whisk. Do not stop stirring! Continue scraping and mixing until the roux becomes the color of coffee with milk.
Whisk in 6 cups of the reserved cooking liquid to start. Whisk until smooth and then add the greens mixture. Add more liquid as needed to keep greens just covered, but not so much that the greens are floating around!
Bring to a boil, then reduce heat and simmer for 20 minutes, stirring occasionally. Stir in parsley and cayenne pepper. Cook for an additional 40 minutes, adding water as needed to maintain consistency noted above.
Taste and adjust salt, and add Chrystal Hot Sauce, and/or lemon juice to taste. If using, add filé powder just before serving.
Serve spooned around a round mound of cooked rice, with Chrystal Hot Sauce and lemon wedges on the side. Pairs nicely with some sazerac or Abita beer.