Preheat the oven to 400 degrees F. Preheat the grill. Toss the carrots, parsnips, and sweet potatoes with 1 tablespoon olive oil. Season with salt and pepper. Place the vegetables on a baking sheet and roast until tender, about 40 minutes. Place the potatoes in a saucepan and cover with water. Season with salt. Bring the liquid to a boil and reduce to a simmer. Cook until the potatoes are fork tender, about 8 to 10 minutes. Drain the potatoes and place back in the saucepan. Add the roasted vegetables to the potatoes. Using a hand masher, mash the potatoes and roasted vegetables together. Season with salt and pepper. Add the cream and 2 tablespoons of the butter. Mix well. Season with salt and pepper. Season the portobellos with the remaining olive oil, salt and pepper. Grill for 3 to 4 minutes on each side. Remove from the grill and set four of the mushrooms aside. Spoon a quarter of the potatoes in the cap of the remaining mushrooms. Spread the potatoes evenly. Place the remaining caps on top of the potatoes, forming a sandwich. Place the portobello sandwiches on a baking sheet and cook for about 6 minutes. In a large saute pan, heat the remaining olive oil. Add the onions. Season with salt and pepper. Saute for 2 minutes. Add the mushrooms and garlic. Season with salt and pepper. Saute for 3 to 4 minutes. Add the parsley and veal reduction. Bring the liquid to a boil and reduce to a simmer. Cook for 2 minutes. Remove from the heat and stir in the remaining butter. To assemble, spoon the mushroom stew in the center of each plate. Place the portobello sandwich in the center of the stew. Garnish with the cheese.