2 pounds mixed mushrooms (such as shiitake, cremini or oyster) , thickly sliced
2 shallots , sliced
5 sprigs fresh thyme
2 tablespoons extra-virgin olive oil
1/2 teaspoon coarse sea salt , divided
1/2 teaspoon ground black pepper , divided
3/4 cup stout beer
2 tablespoons firmly packed light brown sugar
1 tablespoon Dijon mustard
4 skinless, boneless salmon fillets
Preheat the oven to 400°F.In a large bowl, combine mushrooms, shallots and thyme sprigs. Drizzle with oil, season with 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper and toss to combine. Arrange mushrooms in a single layer on a large rimmed baking sheet. Roast, stirring 2 or 3 times, until nicely browned, about 45 minutes. Let cool slightly.
Meanwhile, in a small heavy saucepan, stir together beer, brown sugar and mustard and bring to a boil. Reduce heat and simmer until mixture just starts to thicken, 5 to 8 minutes. Remove from heat and set glaze aside. (Reserve all but 1/4 cup of the glaze for serving.)
Pat fish dry and season with remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Place skin-side down in a large baking dish. Toss mushrooms with 2 tablespoons of the glaze and spoon them around fish. Brush fish with additional 2 tablespoons of the glaze. Roast until fish is just cooked through and flakes easily, 10 to 12 minutes. Serve each piece of fish with a generous portion of mushrooms and reserved glaze on the side.