Place the dried mushrooms into a bowl or large jug, pour over 1½ cups/400ml water, and leave to soak overnight.
Place the pearl barley into a suitable-sized bowl or jug, and add enough water to cover by 2-3” (5-8cm). Leave to soak overnight.
In a medium-large skillet, heat half of the butter over a medium heat, and gently toast the barley for a few minutes, until it starts to turn golden. Keep it moving around the pan to prevent it burning.
Stir the mushrooms into the barley, and add 1 cup (250ml) of the mushroom soaking water.
When the barley and mushrooms have around 10-15 minutes to go, pre-heat your oven to 350F/180C.