Salty, tangy preserved lemon plays two parts here: it's roasted on the same pan as the potatoes, along with garlic, jalapenos, and red onion, and it's used to amp up the parsley-and-mint salad that's sprinkled on just before serving.
Place potatoes in a large pot; cover with water by 2 inches. Bring to a boil, then reduce heat and simmer until just tender when pierced with the tip of a knife, 5 to 6 minutes. Drain and pat dry.
Preheat oven to 425 degrees. Toss potatoes with 1/3 cup oil on a rimmed baking sheet, season with salt, and roast, tossing occasionally, until golden brown and crisp, about 40 minutes. Add jalapenos, garlic, onion, and 7 tablespoons preserved lemon. Cook, tossing once, until onion and jalapenos are softened, about 8 minutes. Season with salt.
Meanwhile, toss parsley, mint, and lemon juice with remaining 1 teaspoon oil and remaining 1 tablespoon preserved lemon. Season with salt and pepper. Serve potatoes warm or at room temperature, with herb salad sprinkled over top.