Roasting the mushrooms gives them a wonderful depth of flavour and also makes them slightly drier, which is best when making these sandwiches.
2 onions, thinly sliced
400 grams portobello mushrooms, sliced
2 tablespoons olive oil
3 big handfuls spinach leaves
8 slices sourdough bread
butter for spreading
100 grams gorgonzola or other soft blue cheese
1 large ball fresh mozzarella in whey, drained and sliced into 8
sea salt and ground pepper
Preheat the oven to 160°C fan bake.
Toss the onions and mushrooms with the olive oil on a large baking tray and season generously.
Roast for about 20 minutes, turning once until tender. Cool.
Put the spinach in a large heat-proof bowl and cover with boiling water, turning to wilt. Drain and pat between paper towels to remove excess water. Chop roughly and add to the mushrooms.
To assemble: Spread one side of the bread with butter. Turn 4 slices over and top with the blue cheese. Spoon on the mushroom mixture then top with two slices of mozzarella. Season and top with the remaining bread, buttered side up.
Heat a sauté pan and cook both sides of the sandwiches until golden. Do this in batches if needed. Transfer to a lined baking tray and place in the oven for a few minutes to finish melting the cheese.
To serve: Cut in half and serve immediately. Makes 4 sandwiches