In a 33 x 23-cm (13 x 9-inch) baking dish, spread a quarter of the béchamel sauce. Cover with a layer of noodles, and top with half the fish. Season with salt and pepper. Add a quarter of the sauce and cover with a layer of noodles. Spread the mashed peas and sprinkle with the mozzarella. Cover with a layer of noodles and add the remaining fish. Season with salt and pepper. Add a quarter of the béchamel. Continue with the remaining pasta and béchamel. Sprinkle with the Parmesan and breadcrumb mixture. Bake for about 40 minutes or until the crumbs are golden brown. Finish cooking under the broiler, if needed. Let rest for 10 minutes before serving.