Tuna-and-Potato Salad with Sorrel Vinaigrette

In a small nonstick skillet, heat 1 tablespoon of the olive oil. Add the onion and cook over low heat, stirring occasionally, until very soft but not browned, about 12 minutes. Scrape into a blender and let cool.

To the blender, add the cucumber and sorrel and puree until smooth. With the machine on, drizzle in the remaining 1/4 cup of olive oil and the canola oil. Strain the vinaigrette into a small bowl, pressing on the solids. Whisk in the crème fraîche and season with salt. Cover and refrigerate.

In a medium saucepan, cover the potatoes with 2 inches of water and season with salt. Bring to a simmer and cook until tender, about 30 minutes. Drain and let cool slightly. Thinly slice the potatoes and transfer to a medium bowl. Toss with the olive oil and season with salt.

In another bowl, combine the sliced cucumber and arugula with 3/4 cup of the vinaigrette and season with salt and pepper; toss to coat. Mound the salad on plates and top with the potatoes and tuna. Garnish with pepper and serve the remaining vinaigrette on the side.