Bring a pot of salted water to a boil. Add the rigatoni and 2 sprigs of thyme and cook, stirring occasionally, until just tender, 8 to 10 minutes. Drain, reserving 1 cup cooking water. Discard the thyme sprigs. Cover the rigatoni and keep warm. While the rigatoni cooks, toast the pine nuts.

In a dry frying pan over medium heat, toast the pine nuts, stirring frequently, until fragrant and beginning to brown, 4 to 6 minutes. Transfer to a small bowl to cool. Meanwhile:

• Remove the root end and any tough green portion of the leek. Cut the leek in half lengthwise then crosswise into thin half-moons slices. Rinse and pat dry.
• Coarsely chop the garlic.
• Pick the thyme leaves from the remaining sprigs and coarsely chop. Discard the stems.
• Zest and juice the lemon.
• Rinse the beans.
• Crumble the ricotta salata.

In the same pan used to toast the pine nuts, over medium-high heat, warm 1 tablespoon oil until hot but not smoking. Add the leeks, garlic, and as much Aleppo pepper as you like. Season with salt. Cook until the leeks are tender, 5 to 6 minutes. Add the beans and chopped thyme and cook until the beans are warmed through, 2 to 3 minutes.

Add the drained rigatoni to the beans with ¼ cup reserved cooking water. Add the arugula and toss until wilted, 1 to 2 minutes. If the pasta seems dry, add more cooking water. Remove the pasta from the heat. Stir in the lemon zest, juice, and the toasted pine nuts.

Transfer the pasta to individual bowls. Garnish with the crumbled ricotta salata and serve.