1 Make the mayonnaise: In a small bowl, stir the mayonnaise and tarragon together. Cover and keep in the refrigerator until ready to serve with the fish cakes.
2 Cook and mash the potatoes: Place the potatoes in a saucepan and cover with cold water. Add 1 teaspoon salt. Bring the potatoes to a boil, adjust the heat to a simmer, and cook for 12 to 14 minutes, or until the potatoes are tender when pierced with the tip of a knife.
In a colander, drain the potatoes and return them to the pot. With a potato masher, coarsely mash them. Set aside.
3 Poach the fish: Sprinkle the fish on both sides with salt and pepper. In a wide skillet, place the onions, bay leaf, and thyme. Place the fish on top of the onions. (If your fish still has the skin on it, place it skin side up.) Add 1/2 teaspoon salt and enough water to barely cover the fish. Cover with a lid.
Over medium heat, bring the water to a simmer. Simmer the fish for 2 to 3 minutes. The fish may be done at this point, but if not, set it aside, covered, for 5 to 6 minutes, or until it is opaque. The cooking time will vary according to the thickness of the fish. Transfer it to a plate to cool. With a fork or your fingers, break the fish into large flakes.
4 Make the fish cake mixture: In a large bowl, beat the eggs. Add the pepper, lemon zest, lemon juice, parsley, additional fresh herbs of your choice, mustard, capers, and scallions. Mix to blend them. Add the mashed potatoes and stir until the ingredients are combined. Add the fish and panko, and stir to combine.
5 Form the patties: Divide the mixture into 8 portions and shape them into patties that are about 3 inches wide and 1-inch thick. Refrigerate, covered with plastic wrap, for at least 1 hour or for up to 1 day.
6 Cook the fish cakes: Set a rack 4 inches from the broiler element and preheat the boiler. Line a baking sheet with foil and brush generously with oil. Place the patties on the baking sheet and brush the tops with oil. Broil for 5 to 6 minutes, or until golden brown.
Remove the pan from the oven and flip the patties over. Brush the tops with more oil. Broil for another 5 to 6 minutes, or until browned. Serve hot with tarragon mayonnaise or tartar sauce.