How to Make It
Cook bucatini according to package directions, adding table salt to the water. Drain pasta, reserving 1½ cups cooking water.
Place tuna, mayonnaise, anchovies, capers, lemon zest and juice, ¼ cup of the olive oil, ½ teaspoon of the kosher salt, and ¼ teaspoon of the pepper in a blender; process until smooth. Place tonnato sauce in a bowl; set aside.
Heat remaining 2 tablespoons olive oil in a large skillet over medium-high. Add onion and celery; cook, stirring often, until vegetables are tender, about 5 minutes. Add garlic; cook, stirring often, 30 seconds.
Stir in beans and pasta; cook 3 minutes. Add tonnato sauce and 1 cup of the reserved cooking water, and toss to coat. If necessary, add remaining cooking water, ¼ cup at a time, until a smooth sauce forms. Stir in chives and remaining ½ teaspoon kosher salt and ¼ teaspoon pepper. Divide among 4 bowls, and top with Olive Oil Breadcrumbs. Serve immediately.