Heat a griddle pan until very hot, and fry the mushrooms first. Add to the pan closed cup side first. Turn after about 5 mins (liquid will have gathered around the stalk). Fry on other side for 5 mins, then keep warm while you cook the halloumi until black marks appear on both sides.
Toast the bread and spoon a little oil on all pieces of ciabatta to moisten. Top with slices halloumi and whole mushrooms. Fill the mushroom cup with dressing and drizzle the rest liberally.