Place clay pot or covered roasting pan into oven to preheat.
Place 3 ounces fish sauce and 1/2 cup sugar in a small saucepan, and use whisk to blend until sugar dissolves. Heat over medium-low heat, whisking until mixture comes to a boil, then stop stirring, and swirl mixture in pan for 12 minutes or so, until it becomes golden, foamy, and slightly thick.
Remove from heat, and when mixture has cooled and foam settled somewhat, stir in lime juice until sauce is blended.
In the bottom of the clay pot or roaster, place oil, shallots, garlic, and chopped lemongrass.
Cover clay pot or roaster and cook at 450°F for 10 minutes.
Remove pot from oven and add catfish fillets in a single layer. Cover pot and cook at 450°F for 5 minutes.
Remove pot from oven and add mushrooms and caramel sauce. Cover pot and cook at 450°F for 5 minutes.
Mix together remaining fish sauce, broth, water, and sugar until blended.
Remove pot from oven and add broth mixture and season to taste with freshly ground black pepper. Return pot to oven and cook, UNCOVERED, for 10 minutes at 450°F.
Remove pot from oven and cover fish and mushrooms with spinach, basil, and chopped scallion. Cover pot and cook at 450°F for 5 minutes.
Remove from oven and serve fish and vegetables with hot steamed jasmine rice.