Toss the portobellos in a small Ziploc bag with the olive oil, garlic, 1 teaspoon of the rosemary, 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 GRAVY: Deglaze the pan (fancy way of saying get up those browned bits that contain so much flavor) by adding the sherry to the skillet, and stirring up any browned bits. Cook until the sherry has been reduced in half. Add the vegetable broth and simmer for five minutes. Stir in accumulated mushroom liquid and remaining chopped rosemary. Whisk in the cornstarch mixture, allow to thicken. Turn the heat to low and whisk in the butter, 1 tablespoon at a time. Add sliced mushrooms. Season to taste with salt and pepper.