Preheat the oven to 450°F
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 potatoes and return to the pot. Mash with a potato masher. Quickly fold in the cream cheese, heavy cream, butter, truffle oil and sour cream and season to taste.
On a baking sheet, toss the corn, fennel, oil, salt and pepper. Roast for 10-15 minutes, until softened.
Preheat the grill or a large skillet over high heat. Pat the steaks dry and season liberally with salt and pepper. Grill the steaks for 3 to 4 minutes per side to sear, then lower the heat to medium and cook until an instant-read thermometer reaches 130°F for rare, 140°F for medium rare and 150°F for medium-well.
Transfer the grilled steaks to a cutting board, let rest for 5-10 minutes, then slice against the grain.
Combine all of the vinaigrette ingredients in a bowl and season to taste.
Divide the mashed potatoes between plates. Top with steak, roasted corn and fennel and tomato vinaigrette.