For the mushrooms: Toss the portobellos in a small Ziploc bag with two tablespoons of the olive oil, garlic, 1 teaspoon of the thyme, salt & pepper. Allow to marinate for at least 3 hours.
Preheat oven to 500 degrees. Place the mushrooms in a lightly oiled oven safe skillet rounded side up. Roast for 6 minutes. Turn and roast for another 6 minutes.
Carefully remove pan from the oven. Carefully reserve the liquid on top for the gravy and set mushrooms aside on a cutting board. Once cooled slightly, slice on a bias into ½ inch thick slices.
For the sauce: Over a medium-high heat, add the remaining olive oil to the now empty (but not cleaned) skillet. Add the shallot and cook until softened, about 2 minutes. Stir in the wine, scraping up any browned bits. When reduced to half, add the vegetable broth and brown sugar. Add in the mushrooms and any accumulated mushroom liquid. Simmer for five minutes. Add remaining chopped thyme. Whisk in the cornstarch mixture, allow to thicken.
Turn the heat to low and whisk in the butter one piece at a time. Off the heat, season with salt and pepper to taste.
