Remove crusts from the bread, break into pieces, and process in a food processor until you get a fine bread crumb. In a large bowl, flake apart the salmon with a fork. Add the egg and mix well. Finely chop 4 of the scallions and add to the bowl. Add the water chestnuts, cilantro, pepper, and the bread crumbs and mix well. Shape the mixture into 12 patties.
In a large nonstick skillet, heat 1 1/2 teaspoons of olive oil over a medium heat. Add 6 patties and cook for 5 minutes on each side. Transfer the cooked patties to a plate and cover with foil to keep warm. Add the remaining 1 1/2 teaspoons olive oil to the pan, and cook the rest of the salmon cakes, 5 minutes on each side.
Chop the remaining 1 scallion. Serve salmon cakes with the sauce and garnish with scallion.
If using regular yogurt place the yogurt in a strainer lined with a paper towel. Put the strainer over a bowl and place in the refrigerator to drain and thicken for 30 minutes.
Place drained yogurt or Greek-style yogurt into a small bowl. Add mayonnaise, ginger, sesame oil, and soy sauce. Whisk until smooth.
Yield: about 2/3 cup