Make the caramel: in the clay pot, combine sugar with 2 tablespoons water over medium heat. Swirl occasionally until it turns deep amber, 5-7 minutes.
Add oil, shallot, and garlic to the caramel; stir 30 seconds. Pour in remaining water and fish sauce, stirring to dissolve.
Add black pepper and chili if using. Taste and adjust: salty-sweet with a bitter caramel edge.
Nestle fish pieces into the sauce in a single layer; spoon sauce over top. Add coconut soda if using.
Bring to a bare simmer, cover, and cook on low 30-35 minutes, basting every 10 minutes, until fish flakes and sauce coats a spoon.
Uncover for the last 5 minutes to thicken if needed. Garnish with green onions and black pepper. Serve hot with steamed rice.