Heat the butter and olive oil in a large frying pan over a medium heat. Dust the hake in the flour, shaking off any excess. When the butter begins to foam, add the hake to the pan. Keep aside 1/2 tbsp of the flour.
Fry the fish for about 2 minutes until sealed and golden then carefully turn and fry for another 2 minutes. Remove and set aside.
Add the garlic and fennel seeds to the pan and fry until golden.Stir in 5 tbsps of the parsley and when it turns slightly darker turn heat to low.
Stir in the reserved flour and cook for 30 seconds, then whisk in the wine and fish stock, slowly at first.
Return the fish to the pan and add the clams ( if using) and peas. Bring to a gentle simmer, then cover and cook for a couple of minutes until the fish has cooked through and the clams are all open.
Sprinkle on the remaining parsley and serve immediately.