Heat the oil in a large pot over medium heat. Add the onion, garlic, and ginger, then cook for 3 minutes until the onion softens.
Stir in the Thai red curry paste and cook for 1 minute to release its aroma.
Add the red bell pepper slices and cook for another 2 minutes.
Pour in the coconut milk and vegetable broth. Stir well and bring the mixture to a gentle simmer.
Add the fish chunks to the pot. Season with salt and pepper, then simmer for 6 to 8 minutes until the fish is cooked through and flakes easily.
Remove the pot from the heat. Stir in the lime juice and most of the cilantro.