Heat olive oil in a large pot over medium heat. Add the chopped onion and sauté until soft and translucent, about 5 minutes. Add minced garlic and grated ginger, cooking for another 1-2 minutes until fragrant. Stir in the sliced red bell pepper and cook for 3 minutes until slightly softened. Pour in the coconut milk and fish or chicken broth, stirring to combine. Bring the soup to a gentle simmer. Add the white fish pieces to the pot and cook for 5-7 minutes or until the fish is opaque and cooked through. Stir in the lime juice, fish sauce, sugar, salt, and pepper. Adjust seasoning to taste. Remove from heat and stir in chopped fresh cilantro. Serve hot with lime wedges on the side.