Preheat the oven to 190°C.
If you don’t have pre-cooked potatoes, cook them in boiling water, drain and let them cool.
Cut the potatoes into chunks and scatter them into a baking tray.
Mix the mustard glaze, and brush the potatoes. Put the tray in the oven and cook for 10 minutes.
Brush the salmon fillets with the glaze and set aside.
Slice the bell pepper and spring onions. Cut the bigger mushrooms in half.
Take the tray out of the oven and scatter the chopped veggies over the potatoes.
Add the thyme leaves, and brush everything again with the mustard glaze.
Mix well.
Put the marinated fish on top.
If you have any mustard sauce left, pour over the dish. Season with salt and freshly ground pepper.
Bake for another 15 minutes or so until the fish has just cooked through.
Serve with fresh herbs scattered on top.