Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until it softens, about 5 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Add the chopped tomatoes, salt, and black pepper. Cook for 3 minutes until the tomatoes begin to break down.
Pour in the fish stock and bring the mixture to a simmer.
Add the diced russet potatoes. Simmer for 15 minutes until the potatoes are almost tender.
Stir in the green beans and continue simmering for 5 minutes.
Gently place the cod chunks into the pot. Cover and cook for 6 to 8 minutes until the fish flakes easily.
Remove the pot from the heat and sprinkle with the chopped parsley before serving.