Cook the rigatoni in a large pot of salted boiling water, per package instructions, until cooked to al dente. Reserve ⅓ cup cooking water then drain the remainder.
While the pasta is cooking, heat the olive oil in a large skillet over medium heat. Add the onion and crushed red pepper flakes then cook, stirring often, for 6-7 minutes, or until starting to caramelize.
2a. Add the white beans.
Add the minced garlic and cook, stirring constantly, for 1 minute.
Add the lemon juice and butter then remove from the heat; whisk until well combined.
Add the cooked & drained pasta, spinach & arugula, and pine nuts.
Season well with sea salt and freshly cracked pepper, to taste. Toss until evenly coated, adding reserved pasta water as needed.
Pour into a serving bowl and top with lemon zest and serve with freshly grated Parmesan, to taste. Enjoy.
