Preheat oven to 375 degrees F (190 degrees C). Spray a cookie sheet with non-stick cooking spray.
In a shallow bowl, marinate mushrooms in Italian dressing for 20 minutes. Drain well.
In a small mixing bowl, combine crabmeat, 1/2 cup of breadcrumbs, eggs, mayonnaise, onions, and lemon juice. Fill the mushroom caps with the mixture. Arrange the mushrooms on the prepared baking sheet. Top with remaining breadcrumbs.