For the greens:.
Rinse and trim greens, removing any dried-out parts or tough stems that don’t break easily. If you are using collards, remove the tough inner rib that runs up the center of each leaf.
Fill the sink with cold water and submerge all greens. Swish them around then leave undisturbed for about 5 minutes, then lift from the water and place in a colander. (Don’t drain the sink with the greens still in it: Soaking the greens allows all the sand and grit to settle to the bottom of the sink—if you drain it, your greens are left sitting in the silty stuff.) Repeat this process until no grit is left in the sink when the water is drained (may take 3 to 4 swish/soaks - do this because gritty greens are AWFUL).
Chop or tear greens into large pieces and place in a large saucepan or pot with a tight fitting lid. Add water to greens and season generously with salt; place over medium-high heat. When the water in the pot begins to simmer, tightly cover and reduce heat to medium low. Cook greens, occasionally turning with a pair of tongs, until they are very soft and wilted, about 15 to 20 minutes.
Strain greens, being sure to reserve the cooking liquid. (You should have about 3 1/2 cups.) Allow greens to cool slightly, then chop into 1/2-inch pieces. Take about 1/2 of the chopped greens and puree them in a food processor or blender (if greens will not blend, add a little of the reserved cooking liquid to help them along).
For the gumbo base:.
In a large, heavy pot or Dutch oven, heat vegetable oil over medium heat. When it is hot, slowly sprinkle in flour, stirring constantly with a wire whisk to prevent any lumps from forming. Reduce heat to medium low and cook roux, stirring constantly (and taking care to scrape out the corners of the pan), until it is a nutty brown color (the color of peanut butter) and emits a toasted aroma, about 10 to 15 minutes.
Use a wooden spoon to stir onion, scallions, bell pepper, celery, and garlic into the pot. Season with a pinch of salt and freshly ground black pepper. Cook, stirring often, until vegetables are softened and garlic is no longer raw-smelling, about 5 minutes.
Add reserved cooking liquid along with vegetable broth or water, stirring well to incorporate. Increase heat to medium high and bring mixture to a simmer. Stir in salt, Cajun seasoning, cloves, allspice, and bay leaves and simmer, stirring often, until gumbo base is soupy and thick and vegetables are tender, about 15 minutes.
Stir in chopped and pureed greens and marjoram; cover the pot and simmer 10 minutes. Add Tabasco to taste and serve over cooked white rice, garnished with thinly sliced scallion tops.