Mix together the tomato, the scallion and the cilantro. Season to taste with salt and pepper.
Spread each of the 12 sourdough slices with one ounce of goat cheese. Top six of the slices with a slice of smoked turkey. Top each turkey slice with 1/6 of the tomato mixture, spread evenly. Top that with a thin layer of avocado slices, seasoned with salt and pepper. Top with the remaining slices of turkey and cover with the remaining 6 slices of bread (which have been spread with goat cheese). Cut each sandwich in half and serve immediately.
As a picnic item, or as a lunch: One sandwich per person is ideal. If it's convenient to serve soup, a mug of Creamy Lentil Soup with Ancho Chiles would be a great accompaniment.
As a buffet item: You could cut these sandwiches in quarters, place them on lettuce-lined platters, and serve small bites to 24 people.