Unless you're approaching the proportions used for pickles (i.e. literally swimming in brine), it's just a contained burst of acid, salt, and mulled seasonings that together work background magic. Used in tablespoons, not pints, it doesn't announce itself, but somehow makes the butter and mushrooms speak louder and more clearly. Adapted slightly from Food & Wine and State Bird Provisions —Genius Recipes
mixed mushrooms, such as cremini, oyster, and stemmed shiitake, thickly sliced or quartered
brine, strained from a jar of dill pickles
In a very large skillet, melt 3 tablespoons of the butter in 2 tablespoons of the olive oil over moderately high heat, swirling, until the butter is golden, about 2 minutes. Add 1/3 of the shallots to the skillet and cook, stirring, until softened, about 1 minute. Add 1/3 of the mushrooms and cook, stirring occasionally, until tender and golden, 5 to 7 minutes. Add 1/3 of the pickle brine and cook until absorbed, about 1 minute. Transfer the mushrooms to a serving bowl and keep warm.
Repeat the process 2 more times with the remaining butter, olive oil, mushrooms and pickle brine. Season the mushrooms with salt and pepper to taste and serve.