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Make the lentils: In a small saucepan, combine the lentils, salt, bay leaf, and 2 cups water. Bring to a boil over high heat, then lower to a simmer and cook, stirring occasionally, until the lentils are tender, 25 to 30 minutes. Turn off the heat, cover, and let rest for 5 minutes. Discard the bay leaf. Drain the lentils, spread on a plate to cool, and drizzle with olive oil.
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Make the vinaigrette: In a small bowl, whisk the egg yolk with the vinegar, mustard, and 2 tablespoons water. Whisk in the olive oil.
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Preheat the oven to 450ºF. Line a rimmed baking sheet with parchment paper.
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Season each salmon fillet with 1/4 teaspoon salt and a few grinds of pepper and place on the prepared baking sheet. Roast until opaque on the outside and just beginning to flake at the edges with a center that is springy to the touch, 7 to 8 minutes. Immediately transfer the fillets to a plate to cool. When cool enough to handle, remove and discard the skin from each fillet and break the fish into large chunks.
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In a large bowl, combine the lettuces with two pinches of salt, the olive oil, and the lemon juice and toss.
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To serve, divide the lentils among four plates. Top with lettuces and salmon chunks. Drizzle a few spoonfuls of the vinaigrette over each salad, sprinkle with tarragon leaves and radishes, season to taste with salt, and serve.