Salt the cod fillets, drizzle with lemon juice, and let marinate for 5 minutes.
Heat a large saucepan over medium heat and add butter to coat the pan. Add the thawed peas and sliced carrots. Sprinkle with salt and white pepper to taste. Pour in the meat stock. Place the cod fillets on top of the vegetables. Cover and cook for 15 minutes.
While the fish cooks, melt butter in another saucepan over medium heat. Add the drained mushrooms and asparagus tips. Stir occasionally and heat for 3-5 minutes until warmed through. Season with salt and white pepper to taste. Fold in the chopped parsley.
Arrange the cooked cod fillets and vegetables on a serving platter. Pour the mushroom-asparagus mixture over them. Serve with boiled potatoes.