Clams with White Beans and Pasta

Name: Clams with White Beans and Pasta
Ingredients: 2 tablespoons extra-virgin olive oil, plus more for drizzling; 2 medium leeks, halved lengthwise and thickly sliced (white and light green parts only); 2 stalks celery, sliced, plus 1/4 cup chopped celery leaves; Kosher salt and freshly ground pepper; 1/2 teaspoon saffron threads; 3 wide strips lemon zest, plus the juice of 1/2 lemon; 1/2 cup dry white wine; 1 cup bottled clam juice; 1 15.5-ounce can cannellini beans, drained and rinsed; 6 ounces ditalini or small shell pasta (about 1 1/2 cups); 24 small littleneck clams; 1/4 cup chopped fresh parsley