5 ounces mushrooms, such as portobello, button or cremini
1 1/2 to 2 pounds white-fleshed fish fillets, such as black cod or Pacific halibut
2 tablespoons unsalted butter, plus an optional 2 tablespoons for swirling in at the end
2 to 3 shallots (or 1/4 small onion), peeled and minced
Salt and freshly ground black pepper, to taste
1 cup dry sparkling hard cider
1/4 cup apple brandy, such as Calvados or applejack (optional)
3/4 cup creme fraiche (optional)
Stem, wash and coarsely chop the mushrooms; there should be 1/2 to 3/4 cup. Cut the fish into 2 or 4 equally sized pieces, if necessary, so that they fit comfortably in a large skillet. Season fish lightly on both sides with the salt and pepper.
Melt 2 tablespoons of the butter in a large skillet, preferably nonstick, over medium-low heat. When it is foamy, add the shallots and cook, stirring occasionally until they are translucent.
Arrange the fish pieces in a single layer in skillet. Cook for 1 to 2 minutes, then add mushrooms, stirring to combine. Add the cider and apple brandy, if using; increase heat to medium or medium-high so that bubbles appear at skillet's edges. Cover and cook for 8 to 10 minutes; turn fish pieces over halfway through.
Transfer fish to a deep serving dish to keep warm, leaving liquid in skillet. Increase heat to medium-high, if needed; cook uncovered for 2 to 3 minutes to reduce sauce slightly. Taste and adjust seasoning as necessary.
(At this point, if you want to make a richer sauce, remove skillet from heat. Add the creme fraiche or butter and stir to incorporate.) Spoon sauce over fish. Serve immediately.
Makes 4 servings.