Heat the olive oil in a large pot over medium heat. Add the onion and fennel and cook for 4 minutes until they soften.
Stir in the garlic, oregano, and red pepper flakes. Cook for 1 minute until fragrant.
Pour in the white wine and let it simmer for 2 minutes to reduce slightly.
Add the crushed tomatoes and fish stock. Bring the mixture to a gentle boil, then lower the heat and simmer for 8 minutes.
Season the halibut pieces with salt and black pepper. Add them to the pot and simmer for 5 to 6 minutes until the fish turns opaque and flakes easily.
Remove the pot from the heat. Stir in the chopped parsley and adjust seasoning if needed.
Ladle the stew into bowls and serve with lemon wedges on the side.