Make the fish mixture by pulsing the onion, garlic, and herbs in a food processor until finely chopped. Transfer to a bowl and combine with the ground fish, lemon zest, chili, olive oil, and spices. Mix well, cover, and refrigerate for 30 minutes.
Shape the chilled mixture into balls. Peel the potatoes and cut into ½-inch/1 cm slices. Using a small knife, make a slit in each slice without cutting all the way through, creating a pocket. Fill each pocket with a fish ball and press to seal.
Arrange the stuffed potatoes snugly in a wide, oven-safe pot. Scatter the bell pepper strips, garlic cloves, and whole chilies over the top. Pour in the sauce, adding more water if needed so the liquid nearly covers the potatoes. Sprinkle with half the cilantro. Cover and cook over medium heat until the potatoes are tender, about 35 minutes.
Remove the lid and transfer the pot to a 375°F/190°C convection oven. Cook uncovered until the tops are browned, about 20 minutes.
Sprinkle with the remaining cilantro and serve hot.