Cook the potatoes in a pot of boiling water for 10 minutes until soft. Drain and transfer to a serving bowl.
Meanwhile sauté button mushrooms in olive oil for 3 minutes until slightly golden in color.
Add the crispy fried onion, stir.
Mash the drained potato with the butter and milk until all lumps have been removed.
Add the feta,and sautéed mushrooms and onion, mix until combined. Set aside in warm place.
Combine breadcrumbs, parsley, garlic, lemon zest, and coarse salt a shallow plate.
Brush top of each fillet with olive oil. Press fillets into the crumb mixture. Place fillets in baking dish, crust-side up. Drizzle with some olive oil.
Bake the cod fillets in the oven at 180°C for 12-15 minutes or until firm.
Serve with lemon or lime wedges and feta and mushroom mash.