In a shallow 9x9 dish, lay out portobello mushroom caps. Mix together vinegar, shallot, thyme and pinch of salt. Pour over mushrooms. Cover and marinate for at least thirty minutes.
Heat oven to 350 degrees F. Place a cooling rack on top of a parchment lined cookie sheet. Place mushrooms on top of cooling rack and place in oven. Roast for 5 minutes. Flip and roast another 5 minutes.