In a small bowl, whisk together the vinegar, oil, Worcestershire, thyme, parsley, garlic, lemon juice, and salt and pepper. Put the flank steak in a wide shallow dish and coat with the marinade. Refrigerate for 45 minutes to 1 hour.
Preheat a grill on high.
Remove the outer skin (rind) of the brie and dice it into 1/2-inch cubes. Add them to a small bowl and toss them with the cornstarch. In a medium saucepan over high heat, add the wine and bring to a boil. Add the brie and stir until the cheese has melted. Once melted, reduce the heat to medium-low and stir in the thyme, garlic, nutmeg, salt and pepper, to taste. Cook for 5 to 6 minutes. Remove the fondue from the heat and transfer it to a fondue pot to keep warm.
Remove the steak from the marinade, straining the excess marinade and put it in the middle of the preheated grill. Cook until desired temperature is reached, about 9 to 10 minutes per side. Remove from the grill to a cutting board and let rest for 5 to 10 minutes.
Slice it on the bias and arrange them on a serving platter. Drizzle with fondue sauce or serve it on the side in a fondue pot.