Heat the olive oil in a large pot over medium heat. Add the onion and cook until it softens, about 5 minutes.
Stir in the garlic and thyme and cook for 1 minute until fragrant.
Add the diced tomatoes and cook for 3 minutes, stirring occasionally, until they begin to break down.
Pour in the fish broth and add the potatoes. Bring the mixture to a simmer and cook for 15 minutes until the potatoes are tender.
Season the broth with salt and black pepper. Gently place the sea bass chunks into the pot and simmer for 5 to 7 minutes until the fish flakes easily.
Ladle the stew into bowls and sprinkle with the chopped parsley before serving.