Preheat your oven to 200°C/400°F/gas mark 6.
Roughly chop the potatoes and put them in a large saucepan. Add enough cold water to cover them completely and then bring to the boil until the potato is just cooked through about 3 minutes.
Then, add the asparagus to the saucepan and cook for another 2 minutes.
Drain the potatoes and asparagus in a colander and then leave them to dry for a couple of minutes.
In a bowl combine all the dill mustard sauce ingredients and stir to make a thick paste. Set aside.
Add the olive oil to the sea trout fillets. It should be coated in olive oil. Season the fish with salt and pepper.
Put the boiled potatoes in a large roasting pan, then place in an oven and cook on Gas Mark 7/220c for about 15 minutes until they are golden brown and tender.
Add the trout fillets and asparagus to the roasting pan. Half od the dill mustard sauce is added on top of everything. Roast for another 10-12 minutes or until the trout is just cooked.
Just before you want to serve, fluff the potatoes with a fork and drizzle the remaining sauce. Add the cooked asparagus and sea trout fillets to the dish.
Serving Suggestions: Sprinkle the top with the chopped dill and serve while still hot.