Roasted Potatoes With Anchovies and Tuna

Name: Roasted Potatoes With Anchovies and Tuna
Ingredients: 1 1/2 pounds baby or fingerling potatoes; Kosher salt; Extra-virgin olive oil, for roasting; 6 tablespoons unsalted butter (3/4 stick); 1 (2-ounce) tin anchovies, chopped (about 10 to 12 anchovies); 1 tablespoon capers, drained; 2 garlic cloves, finely grated or minced; 1 (5- to 6-ounce) can or jar tuna, drained and flaked, or 1 (13-ounce) can chickpeas or white beans, drained; 1/2 teaspoon freshly ground black pepper; Fresh lemon juice, to taste; 1 cup chopped soft herbs, including parsley, celery leaves and dill (optional); 1/4 cup thinly sliced red onion, shallot or scallion