canola oil (for preparing the pan)
1 package powdered unflavored gelatin (1/4 ounce)
1/2 cup cold water
3 ripe avocados, preferably Hass (pitted and peeled)
1/4 cup finely minced scallions
  grated zest of 1 lemon
2 tablespoons fresh lemon juice
1/3 cup mascarpone cheese
1/2 cup sour cream
2 tablespoons minced fresh flat leaf parsley or cilantro
  dash Tobasco sauce or cayenne pepper (to taste)
2 teaspoons salt
  Freshly ground black pepper (to taste)
8 leaves Boston (Bibb of butter) lettuce
8 tomato roses (as garnish, optional)
8 sprigs fresh flat leaf parsley (as garnish)
  Croutons or crackers