Trim the greens, removing any ribs and tough stems. Using the kitchen sink or a very large basin, submerge the greens in cold water, swish them around, then let sit for 5 minutes. Lift the greens from the water and place in a colander. Repeat if necessary, draining the sink first and rinsing out the silt and dirt on the bottom of the sink before filling it with water again.
Coarsely chop the greens into large pieces and place in a large pot. Cover with 3 cups of water, sprinkle generously with salt, and bring to a boil over medium-high heat. Reduce heat to medium-low, cover, and gently simmer for 15-20 minutes, turning occasionally with tongs, until tender and wilted. Reserving the cooking liquid, strain the greens and transfer to a cutting board. When cool enought to handle, chop into 1/2-inch pieces. Place half of the chopped greens into a blender or the bowl of a food processor, and purée. Combine the chopped greens and the puréed greens in a large bowl, and set aside.
Heat the vegetable oil in a large, heavy bottomed pot or Dutch oven over medium heat. When hot, slowly whisk in the flour, whisking constantly to avoid lumps. Reduce heat to medium-low, and cook, stirring constantly, for 10-15 minutes, until the roux is fragrant and is the color of peanut butter. Be sure to scrape out the edges of the pot. Stir in the onions, white and light green parts of scallions, bell pepper, celery, and garlic. Sprinkle with salt and pepper. Cook 4-5 minutes, stirring frequently, until the vegetables are softened. Pour in the reserved cooking liquid from the greens, then add the vegetable broth or water. Raise the heat to medium-high, and bring to a simmer. Add the 2 teaspoons of salt, Cajun seasoning, cloves, allspice, and bay leaves. Simmer, stirring occasionally, for 15 minutes, until the vegetables are tender and the liquid has thickened. Stir in the reserved greens, add the marjoram, and cover. Simmer for an additional 10 minutes. Season to taste with Tabasco, and serve over cooked white rice, sprinkled with reserved thinly sliced scallion tops.