Place peppercorns on a cutting board or seal in a resealable plastic bag and coarsely crush with a large skillet or heavy pot. Or coarsely crush in a spice mill. They should be a lot of coarse powder with some larger pieces.
Toss mushrooms, ¼ cup oil, and 2 tsp. salt in a large bowl to combine.
Heat 2 Tbsp. oil in a medium cast-iron skillet over medium-high. Add half of mushrooms and cook, stirring occasionally, until golden brown, about 4 minutes. Transfer to a plate. Repeat with remaining mushrooms and 2 Tbsp. more oil.
Return all mushrooms to pan and reserve plate. Add garlic, and 5 Tbsp. butter and cook, stirring often, until mushrooms are deeply browned and crisp, about 3 minutes. Transfer to reserved plate.
Add shallot and remaining 3 Tbsp. butter to pan; cook, stirring occasionally, until shallot is softened, about 3 minutes.
Remove pan from heat; add cognac. Return to heat and cook, stirring occasionally, until reduced by half, about 4 minutes. Add cream, soy sauce, and mustard and whisk to combine. Bring to a simmer and cook, stirring occasionally, until sauce is thickened to a spoonable consistency, about 3 minutes. Remove from heat.
Add mushrooms and any juices on plate to sauce and toss to coat. Season with salt and pepper. Serve mushrooms with salad and bread alongside.
