1 1/4 cups Marsala wine (see note)
3/4 cup homemade chicken stock or low-sodium store-bought broth
1 packet unflavored gelatin, such as Knox (2 1/2 teaspoons)
6-8 wild Alaska pollock fillets (4-6 ounces), halved and thoroughly dried
Kosher salt and freshly ground black pepper
1 cup all-purpose flour, for dredging
1/4 cup extra-virgin olive oil, plus more if needed
10 ounces cremini mushrooms, stemmed and thinly sliced
4 medium or 2 large shallots, minced
2 medium cloves garlic, minced
1 teaspoon minced fresh thyme leaves
3 tablespoons cold unsalted butter, cubed
1 teaspoon soy sauce
White wine vinegar, sherry vinegar, or fresh lemon juice, to taste
Minced fresh parsley, for garnish
1. Combine Marsala and stock in a mixing bowl or large measuring cup and sprinkle gelatin over surface. Set aside.
2. Season both sides of fish with salt and pepper. Pour a roughly 1/2-inch layer of flour into a wide, shallow bowl. Dredge each fillet in flour, tap off excess, and transfer to clean plate.
3. Heat olive oil in a large skillet over medium-high heat until shimmering. Working in batches if necessary, add fish and cook, turning once, until browned on both sides, about 2 to 3 minutes per side. Using a slotted spatula, carefully transfer to paper towels to drain.
4. Add mushrooms to skillet (do not drain remaining oil; add more oil if pan seems too dry at any point) and cook, stirring frequently, until mushrooms have released their juices and browned well, about 10 minutes. Add shallots, garlic, and thyme and cook, stirring, until shallots are translucent, about 2 minutes.
5. Pour Marsala mixture into pan, making sure to scrape in all the gelatin. Bring to a boil, whisking and scraping up any browned bits from bottom of pan, until liquids are reduced by about three-quarters. Add butter and soy sauce and whisk constantly until emulsified and sauce takes on the consistency of heavy cream. Season with salt and pepper. Taste sauce and adjust with a small amount of vinegar or lemon juice, as needed.
6. Place fillets on serving dish and cover them with the sauce to warm through. Garnish with parsley and serve.