Medium dice the fish. Thoroughly clean station and cooking equipment used for the fish.
Heat oil in a pot over medium heat. Add onion and cook, stirring occasionally, until softened.
Add garlic and poblano. Cook until fragrant.
Add fish, tomatoes and their juices, cilantro, and oregano. Stir to combine. Add up to 1/2 cup water if the stew seems too thick.
Cover and simmer for 20 minutes. Remove from the heat. Season with salt and pepper.
Serve warm or at room temperature, garnished with avocado.