Mushroom-and-Herb Gravy with Apple Brandy

Bring jus to a simmer in a medium saucepan over medium. Reduce heat to low to keep warm. Transfer 2 tablespoons of the warm jus to a small bowl; whisk in 1 tablespoon of the cornstarch to create a slurry. Set aside. Bunch together thyme, rosemary, and parsley, and secure with kitchen twine to create an herb "whisk"; set aside.

Heat a high-sided 12-inch cast-iron skillet or Dutch oven over medium-high. Add oil and 2 tablespoons of the butter; swirl until melted and bubbly. Add mushrooms in an even layer; cook, undisturbed, until starting to brown, about 2 minutes. Stir and agitate mushrooms; cook, undisturbed, until golden brown, about 2 minutes. Stir and continue to cook, stirring occasionally, until deeply browned and liquid has evaporated, about 3 minutes. Add shallots and salt; reduce heat to medium. Cook, stirring occasionally, until shallots soften, about 2 minutes. Remove from heat, and add 1/3 cup of the brandy. Return to heat over medium-low; cook, stirring constantly, until brandy has evaporated, about 30 seconds. Increase heat under skillet to medium; add reserved warm jus (just under 3 cups) in saucepan, and bring to a simmer. Simmer vigorously, stirring occasionally, until slightly reduced, 4 to 5 minutes. Stir in mustard and reserved cornstarch slurry; simmer, stirring constantly, until thickened, about 1 minute. To create a thicker sauce, if desired: Stir together 2 tablespoons of the simmering gravy and 1 to 2 teaspoons additional cornstarch in a small bowl; return to simmering gravy, and boil, stirring constantly, 1 minute.

Remove skillet from heat; add remaining 3 tablespoons butter, 1 piece at a time, and 1 tablespoon brandy. Whisk with reserved herb "whisk" until mixture is melted and creamy, and herbs have infused into gravy, about 2 minutes. Season to taste with additional salt. Transfer to a gravy boat. Serve immediately.