Toss portobello mushrooms in olive oil for grilling and add salt and pepper to taste.
Apply to heat source gill side down until they sweat, about 2 minutes. Turn over and finish cooking. Remove from grill and toss in 1/2 cup balsamic vinegar to marinate. Do not cover: mushrooms will absorb flavor as they cool.
Cook potatoes in cold, salted water. Bring up to a boil and simmer until tender. Strain and add to a mixing bowl. Mash by hand with 3 to 4 tablespoons of butter.
Bring 1 cup heavy cream to a boil and slowly stir into mashed potatoes until smooth . Use as only as much of the heavy cream as you need. Add scallions, garlic and salt and pepper to taste.
In a small saucepan reduce 2 cups balsamic vinegar over low heat until syrupy in consistency. Reserve warm for sauce.
Brush ears of corn with olive oil and add salt and pepper. Grill approximately 4 to 5 minutes, rotating corn to prevent burning. When cool enough to handle remove kernels with knife and reserve for garnish.
On a plate lay a small amount of mesclun in center for underliner, place first portobello down and apply small amount of mashed potatoes as second layer. Repeat until third portobello is top layer, with two layers of mashed potatoes in between. Garnish around plate with roasted corn and chive sticks and red onion. Do not overcrowd plates. Using a spoon drizzle balsamic reduction on top of tower and around plate.