Heat olive oil in a large pot or Dutch oven over medium heat.
Add diced onion and bell pepper; sauté for 5 minutes until softened.
Stir in minced garlic, oregano, and red pepper flakes; cook for 1 minute until fragrant.
Add diced tomatoes (with juice) and fish stock; bring to a gentle boil.
Reduce heat to low and simmer for 10 minutes to allow flavors to meld.
Season the fish chunks with salt and pepper, then gently submerge them into the simmering liquid.
Cover and cook for 5 to 7 minutes, or until the fish is opaque and flakes easily with a fork.
Garnish with fresh parsley and serve immediately.