FOR THE MAYONNAISE (MAKES ABOUT 1 CUP):
SALMON: Generously season the salmon fillet on both sides with the salt and pepper. Place the salmon in an 8-inch skillet. Add water and lemon juice. Bring the water to a simmer over high heat. Reduce the heat to low, tightly cover the pan and poach salmon until medium-rare (5-6 minutes). Remove pan from heat. Allow salmon to cool to room temperature in liquid with the lid on.
Remove the salmon from the liquid and break it apart with your fingers into small pieces. In a medium bowl, combine the poached salmon, smoked salmon and mayonnaise. Mix well. Store tightly covered in the refrigerator. Serve cool, but not cold. Makes about two cups.
MAYONNAISE: In a blender, combine the egg yolk, lemon juice and mustard and blend until well mixed. With the machine running, add olive oil, at first by drops and then, as the mixture emulsifies, in a thin, steady stream until all the oil is incorporated. Add the remaining mayonnaise ingredients and blend. Taste for seasoning. Scrape into a jar, cover and refrigerate until needed (will keep for 2-3 days in refrigerator).
NotesThis is a play on classic rillettes, which is a smooth, savory paste made with salt-cured pork cooked in its own fat and then pulverized. For entertaining, this has countless uses, serve it on olive oil potato chips or raw vegetables such as celery, endive, romaine or radicchio. Drape it over sautéed halibut. It's also great in the center of a bowl of chilled pea soup. For the most luscious texture, poach the salmon on your stovetop's lowest possible setting, with the water just gently steaming.
If you're not sure how to do it, ask the fishmonger to remove the skin and pin bones from the salmon. If you feel up to it, you can remove the pin bones yourself with needle-nose pliers. Skin the salmon by sliding a thin, sharp knife between the skin and the flesh.