Heat the oil in a saucepan over medium heat. Add the garlicand sauté until soft, about 3 minutes. Add the mushrooms and cook for another 5-7 minutes.
In a medium bowl, combine the tomatoes, onions, mushrooms, jalapenos, cilantro and lemon juice. Mix well and season to taste with the salt and pepper.
Allow to stand at room temperature for 15 - 20 minutes to allow flavors to meld. If you don't plan on eating it right away, place it in the refrigerator, covered, until ready to use. Make up to 2 days in advance.