In a skillet, heat the olive oil and butter over medium heat. Add the chopped garlic and onions. Fry for 3-4 minutes, stirring occasionally. Remove from the heat and stir in 3 tablespoons of lemon juice. Add the bread crumbs, oregano leaves, parsley, paprika powder and ground walnuts. Mix well.
Preheat oven to 400 degrees F. Wash the cod fillets and pat dry. Place them in a greased baking dish (or foil roasting pan). Sprinkle with the remaining lemon juice, drizzle a bit of olive oil and season lightly with salt and black pepper on each fillet. Press the bread crumbs mixture over the top of the cod fillets.
Place on the middle rack of the oven and bake until cooked through, about 25-30 minutes. Transfer to a serving plates and serve immediately.