1
Combine parsley, garlic, canola oil, salt, pepper, and lemon juice in a blender and purée to make the gremolata. In a container with a lid, coat the steak with gremolata. Cover and place in the refrigerator overnight or for at least 4 hours.
2
Preheat the oven to 375°F.
3
In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the scallions.
4
Meanwhile, prepare the sauce. Heat the butter in a small skillet and stir in the flour; cook to make a roux, stirring for about 5-10 minutes. Turn off the heat.
5
Heat the olive oil in a separate skillet over medium-high heat. Add the garlic and cook until golden. Add the onions and sauté until translucent. Stir in the thyme and A-1 sauce and heat through. Turn off the heat and pour in brandy, then turn the burners to high and flambé until the alcohol is cooked out. Stir in the mushrooms and cook until their natural water releases. Whisk in the heavy cream, broth and the roux and simmer on low until reduced to your desired consistency. Turn off the heat and whisk in the sour cream. Season to taste.
6
On a baking sheet, toss together the mushrooms, broccoli, garlic, chile flakes, oil and a pinch of salt and pepper. Roast for 12-15 minutes, until crisp-tender.
7
Heat a grill or large skillet to medium-high. Remove the steaks from the marinade, discarding excess marinade and grill for 2 to 3 minutes per side to sear. Transfer to a baking sheet and roast to desired doneness, 8 to 10 minutes (for medium, the internal temperature should be 135°F, 125°F for rare). Rest for 5 minutes before serving.
8
To serve, divide the mashed potatoes and roasted vegetables between plates. Top with steaks and drizzle with sauce.