For the balsamic marinated roast beef: Put the balsamic vinegar, chopped garlic and black pepper in a small saute pan over medium to high heat and reduce for 1 to 2 minutes. Remove the pan from the heat, stir in the chopped rosemary and olive oil, and allow the mixture to cool.
Put the marinade in a baking pan, put the beef in the pan and coat with hands. Put in the refrigerator to marinate for a minimum of 30 minutes, maximum 24 hours.
Preheat the oven to 350 degrees F and remove the marinated beef from the refrigerator. Put the beef in the oven and roast for approximately 45 minutes to 1 hour, until cooked to medium-rare and an instant thermometer inserted in the center of the roast has an internal temperature between 130 and 140 degrees F.
Remove the beef from the oven and allow the beef to rest before slicing.
For the mashed potatoes: Cook the potatoes in enough heavily salted water to cover, until tender. Drain the potatoes, reserving 3 tablespoons of the water. Put the potatoes back into the hot pot and using a potato ricer, rice the potatoes or mash.
Put the butter, olive oil, and reserved potato water in the pot with the potatoes. Season the potatoes with salt. Gently fold the ingredients together until incorporated. Taste and adjust seasoning, if necessary.
For the baked camembert and herb oil: Preheat the oven to 350 degrees F and line a baking dish or oven-proof saute pan with parchment paper.
Put the thyme, marjoram, rosemary, black pepper, and olive oil in a bowl and mix to incorporate.
Put the bottom half of the Camembert on parchment paper. Spoon 1 tablespoon herb oil in the center of Camembert, place the top half on top and pour 1 tablespoon herb oil over top. Place the Camembert in the oven until the cheese has melted, approximately 10 minutes.
For the sauteed mushrooms: Put the remaining herb oil and butter in a saute pan over medium-high heat. Put the mushrooms in the hot pan and saute until slightly caramelized. Season with salt and pepper, and then remove from the heat.
To assemble: To serve, place the roast beef and mashed potatoes on a warm plate and spoon the sauteed mushrooms and baked Camembert over the meat and potatoes.