1) Use a fingernail brush or any such kitchen equipment to brush away the dirt from the mushrooms.
2) Cut the stems of the mushrooms and save them for later use.
3) Keep the caps of small mushrooms whole.
4) Cut the medium sized mushrooms into halves.
5) Quarter the large mushrooms.
6) In a large bowl, mix together lemon juice, garlic, oil, thyme and tomato paste.
7) Toss the mushrooms in the mixture until well coated.
8) Chill the mushrooms for 1 to 2 hours.
9) Serve chilled mixed with salad greens or in sandwich.