Preheat oven to 450°. Toss first 6 ingredients, 3 1/2 tablespoons olive oil, and 1 teaspoon salt together in a large mixing bowl. Spread mixture in a single layer onto a large rimmed baking sheet. Roast 20 minutes or until potatoes and Brussels sprouts are tender and browned, stirring halfway through cooking time. Meanwhile, pat steaks dry with a paper towel. Season steaks on both sides with cracked black pepper and 2 teaspoons salt. Heat 1 tablespoon butter and remaining 2 tablespoons olive oil in a large stainless steel or cast iron skillet over high heat. Add steaks and cook, flipping often, 5 to 6 minutes per side or until a thermometer inserted into thickest portion registers 130° (medium). Remove steaks from skillet; cover loosely with aluminum foil. Reduce heat to medium, and add mushrooms and shallots to skillet; cook, stirring often, 1 to 2 minutes. Add minced garlic, and cook 1 more minute. Add brandy, stirring to deglaze skillet; cook 2 minutes or until reduced by half. Stir in beef stock, mustard, and Worcestershire; bring to a simmer, and cook 3 minutes. Reduce heat to medium, and stir in cream and remaining 2 tablespoons butter until butter melts. Cut steaks into 1/2-inch slices; top with mushroom sauce, and garnish, if desired. Serve with potatoes and Brussels sprouts.