In a large pot, cover the potatoes with cold water and bring to a simmer. Simmer until soft but not mushy, about 16 to 20 minutes. Drain the cooked potatoes and return to the saucepan; place over a medium-low heat for 1 to 2 minutes to dry out the moisture, then mash using a potato masher. Quickly fold in the cream cheese, heavy cream, butter, truffle oil and sour cream and season to taste.
Meanwhile, heat the olive oil for the mushrooms in a large saucepan over medium heat. Add the mushrooms, sherry, and garlic, and sauté until mushrooms are lightly browned, about 5 minutes. Reduce heat to low, stir in the parsley and thyme, and simmer until mushrooms are tender, about 5 to 7 minutes.
For the steak sauce, melt the butter over medium heat, add the shallots and sauté until softened; add the ketchup, Worcestershire sauce, and balsamic vinegar and bring to a simmer. Season to taste.
Preheat the oven to 375°F and heat a grill or large skillet to medium-high.
Lightly brush the steak with oil, season with salt and pepper 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-rare the internal temperature should be 135°F, for rare, 125°F). Rest for 5 minutes before serving.
To serve, divide the mashed potatoes between plates. Top with steak, sautéed mushrooms and drizzle with steak sauce. Garnish with chives.