Combine spinach, mustard greens, beet greens, cabbage, watercress, parsley, green onions, bay leaf and water in an 8-quart Dutch oven over high heat and bring to a boil. Reduce heat to low and simmer, uncovered, for 20 minutes.
Meanwhile, heat oil in a 12-inch skillet over medium heat. Add celery, green pepper and garlic and cook 10 minutes. Add to greens in Dutch oven along with TABASCO® Sauce, salt, basil, thyme and allspice; simmer 40 minutes longer.
Carefully remove 6 cups cooked greens; puree in a food processor or blender in batches and return to Dutch oven.
Melt butter in a 12-inch skillet over medium heat; stir in flour until well blended and smooth. Gradually stir in 2 cups greens mixture and cook until thickened, stirring constantly. Return to Dutch oven and bring to a boil, stirring occasionally. Serve over hot cooked rice, if desired.
Makes 8 servings.