Bring a medium sauce pot of generously salted water to a boil. Add the rigatoni and cook until just tender, 10 to 12 minutes. Drain, reserving ¾ cup pasta cooking water.
While the rigatoni cooks, prepare the remaining ingredients and the sauce.

• Peel and thinly slice the red onion.
• Finely chop, press, or grate enough garlic to measure 1 teaspoon.
• Rinse the white beans.
• Zest and juice the lemon; measure out 1 tablespoon juice.

In a large frying pan over medium-high heat, warm 2 to 3 teaspoons oil until hot but not smoking. Add the onion, season with salt, and cook, stirring occasionally, until softened and beginning to caramelize, 4 to 5 minutes. Stir in the garlic and as much Aleppo chile as you like and cook until fragrant, 1 to 2 minutes.
Add the beans and reserved pasta cooking water and season with pepper. Bring to a boil, reduce to a simmer, and cook until the liquid is slightly thickened, 2 to 4 minutes. Add the rigatoni, arugula, pesto, and the lemon zest and juice, and cook, stirring, until the arugula wilts, 1 to 2 minutes. Remove from the heat and season to taste with salt.

Transfer the rigatoni to individual bowls. Top with the Parmesan and serve.

Chef’s Tip Garlic can turn bitter if it’s cooked for more than 1 to 2 minutes over medium-high heat, so in step 3, be sure to have the white beans and pasta water at the ready before the garlic goes into the pan.