Preheat the oven to 350°F.
Peel the potatoes and slice into 1/4-inch rounds.
Peel the onion and cut into thin strips.
Thinly slice the garlic cloves and, if using, the red bell pepper.
In a large ovenproof skillet over medium heat, heat the extra-virgin olive oil.
Add the potatoes, onion, garlic, and red bell pepper; season with salt and pepper.
Cook over medium-low heat for 15–20 minutes, stirring occasionally, until the potatoes begin to soften.
Use a spoon to make small wells in the vegetables, then place the cod fillets in the skillet, skin side up.
Pour the white wine (or fish stock) and water around the edges of the skillet.
Bake for 15–20 minutes, until the cod is opaque and flakes easily with a fork.
Remove from the oven and sprinkle with chopped fresh parsley.
Serve immediately.