1. Remove crusts from bread, break into pieces, and process in a food processor until they become fine crumbs.
2. In a large bowl, flake apart salmon with a fork. Add eggs and mix well. Finely chop 4 scallions and add to the bowl, followed by water chestnuts, cilantro, pepper, and bread crumbs. Mix well.
3. Shape the mixture into 12 patties. In a large nonstick skillet, heat 1-1/2 teaspoons olive oil over medium heat. Add the patties in batches and cook for 5 minutes on each side.
4. Chop remaining scallion. Serve salmon cakes with sauce and garnish with scallion.
1. If using regular yogurt, place it in a strainer lined with a paper towel. Put the strainer over a bowl and place it in the refrigerator for 30 minutes to let the yogurt drain and thickened. In a small bowl, combine the thickened or Greek-style yogurt with mayonnaise, ginger, sesame oil, and soy sauce. Whisk until smooth.