How to Make It
In a large saucepan, cover the potatoes with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until just tender, about 15 minutes. Drain and let cool slightly, then halve lengthwise.
Meanwhile, light a grill or preheat a grill pan. Brush the scallions with oil and season with salt and black pepper. Grill over high heat, turning, until lightly charred, about 2 minutes. Transfer to a plate, cover with plastic wrap and let steam for 10 minutes. Cut the scallions into 1-inch lengths.
Brush the potatoes with oil and season with salt and pepper. Grill cut side down over high heat until lightly charred, 3 to 5 minutes. Transfer to a plate.
In a large bowl, whisk the 1/3 cup of olive oil with the vinegar and lemon juice. Add the scallions and potatoes and toss well. Season with salt and piment d’Espelette and toss again. Scatter the jalapeño slices on top, garnish with a large pinch of piment d’Espelette and serve warm or at room temperature.