Bring a pot of lightly salted water to a boil over high heat. Fill a large bowl halfway with ice water. Add the snow peas to the boiling water and cook them until just tender, approximately 1 minute. Use a slotted spoon to transfer them to the ice water to stop the cooking and preserve their color.

Drain them, julienne them, and set them aside.

In a bowl, whisk together the egg yolk, 1 tablespoon of the lemon juice, the mustard, garlic, cayenne, and a pinch of salt, either by hand or using an electric mixer. Add the 1 cup olive oil, a drop at a time at first, and then in a thin steady stream, whisking constantly to form an emulsified dressing. Whisk in the lemon zest, the remaining 1 tablespoon lemon juice, and the grapeseed oil. If the dressing is too thick, whisk in a few drops of warm water. Season to taste with salt and pepper and set it aside.

Bring a pot of salted water to a boil. Add the pasta and cook until al dente, 8 to 9 minutes. Drain the pasta and transfer it to a bowl.

Add the watercress to the bowl, along with the remaining 2 tablespoons olive oil. Toss, then let cool at room temperature for 10 minutes. Add the snow peas, scallions, salmon, and enough dressing to coat nicely (You might not use all of the dressing.) Toss, taste, and adjust the seasoning with salt and pepper if necessary Add a squeeze of lemon juice to brighten the flavors, then toss in the herbs.

To serve, divide the pasta among individual warmed dishes, and grind some black pepper over each serving at the table.

VARIATIONS

Various salmon preparations can be substituted for the smoked fish; cured salmon (gravlax) or flaked poached salmon would be equally delicious. Smoked trout or whitefish would also work well.

Fresh green peas or thawed frozen petits pois can be used in place of the snow peas.