2 lbs (900g) mixed mushrooms (king oyster, shiitake, oyster and cremini, sliced or torn)
2 cloves garlic, finely chopped
2 sprigs rosemary
¼ cup (60ml) dry white wine
3 tablespoons (45ml) olive oil
2 tablespoons (30ml) balsamic vinegar
1 tablespoon (15g) grainy Dijon mustard, plus more for serving
Freshly ground black pepper
½ cup (125ml) mayonnaise
2 tablespoons (30g) prepared horseradish
3 tablespoons (5g) finely chopped tarragon
2 tablespoons (8g) finely chopped chives
8 slices sourdough bread, lightly toasted or grilled and rubbed with garlic
2 handfuls arugula
125g brie, thinly sliced, optional
Preheat oven to 450F (232C).
Place mushrooms onto a large rimmed baking sheet (if mushrooms are crowded, use 2 smaller sheets) along with garlic and rosemary.
Whisk together wine, olive oil, vinegar and mustard in a small bowl. Season with salt and pepper. Pour over mushrooms and toss to combine. Spread mushrooms into a single layer. Roast until golden and beginning to become crisp, about 20 to 25 minutes, stirring halfway through.
Mix together mayonnaise, horseradish, tarragon and chives in a small bowl. Season with pepper.
To assemble the sandwiches, spread mayonnaise mixture on one side of four slices of toasted or grilled bread, and one side of remaining slices with Dijon. Layer with arugula, warm mushrooms and brie, if using.