To prepare the burgers: In a medium bowl, cover mushrooms with boiling water and allow them to soak until soft, about 45 minutes. Drain the mushrooms and squeeze out the excess water. Blot the mushrooms with paper towels, cut out the tough stems and discard. In a food processor, process the mushrooms until finely chopped and place them in a medium bowl. Add the remaining burger ingredients and gently mix with your hands. Shape into 4 patties, each about 3/4 inch thick.