Step 1 Heat the oven to 300 degrees. Place the salmon, meat side up, on a cutting board and feel along the surface of the flesh with your fingertips just above and below the midline. If you feel pin bones, pluck them out with tweezers or needle-nose pliers. Season the salmon with salt and place in a jellyroll pan.
Step 2 When the oven is ready, place a baking pan on the floor of the oven and fill it with boiling water. Place the pan with the salmon on the rack. Cook until a paring knife or toothpick penetrates the flesh easily, 25 to 30 minutes. Note that the flesh will be somewhat denser than usual, and it won't change color as much as with other cooking methods.
Step 3While the salmon is cooking, prepare the dressing. Combine the mayonnaise, anchovies, green onion, parsley, tarragon, chives and tarragon vinegar in a food processor or blender or use a mortar and pestle and puree until fairly smooth. There should still be some dots of herbs visible. Set aside.
Step 4Wash the radishes well; the greens have a way of hiding grit. Remove any greens that look wilted or yellowish. Pat dry and, leaving the greens attached, slice the radishes into lengthwise quarters. Pick over the watercress, trimming and discarding any tough stems. Combine the radishes and watercress in a mixing bowl.
Step 5By now the salmon should be done. There will probably be some white collagen collected on the surface. Moisten a pastry brush with oil and lightly brush it away. Dress the radishes and watercress with the olive oil and red wine vinegar and season to taste with salt.
Step 6Serve the salmon on a large platter surrounded by the salad. Serve the dressing in a bowl on the side.