In a large pot or Dutch oven, heat the oil and butter over medium heat.
Once the foam has subsided, add the onion and cook for about 5 minutes, until soft.
Add the red pepper and celery and cook for a few more minutes, until they soften as well.
Add the flour and stir to coat, then add the stock and cream and bring to a simmer, stirring often, until thickened and smooth.
Stir in the potatoes and corn, bring back to a simmer and cook for 15 minutes, or until the potatoes are just tender.
Add the cod—it should cook through in a few minutes.
Stir in the parsley (if you’re using it), and season with salt and pepper.