How to Make It
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.
If sorrel is unavailable, you can substitute 2 cups baby spinach plus 1 tablespoon fresh lemon juice.