Heat oven to 400° F.
Cut the potatoes into French fry-size strips. Place on a baking sheet. Drizzle with the oil, season with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper, and toss. Spread in a single layer. Roast until golden and crisp, 45 to 50 minutes.
Meanwhile, melt the butter in a large pot over medium heat. Add the garlic and cook for 1 minute. Add the leeks and cook until softened, 4 to 5 minutes. Add the wine, tomatoes, and the remaining salt and pepper. Bring to a boil. Cook for 2 minutes. Add the mussels, cover, and cook until they open, about 3 minutes. Discard any mussels that remain closed. Divide the mussels and cooking liquid among individual bowls. Serve with the fries and an extra bowl for the discarded shells.
Tip: Refrigerate the mussels as soon as you get home from the store. If you can't cook them that night, remove the packaging, transfer them to a bowl, cover with a damp cloth, and keep in the refrigerator for no more than 24 hours.