Bring a medium pot of salted water to a boil. Wash and dry all produce. Zest and quarter lemon. Trim and discard woody bottom ends from asparagus; cut stalks crosswise into 1-inch pieces. Peel and mince garlic. Trim and thinly slice scallions, separating whites from greens. Cut 3 TBSP butter into cubes (6 TBSP for 4 servings).

Once water is boiling, add rigatoni to pot. Cook until al dente, 9-11 minutes. Reserve 1 cup pasta cooking water, then drain. Meanwhile, in a small bowl, combine ricotta, lemon zest, a squeeze of lemon juice, and a big pinch of salt and pepper.

Heat a large, dry pan over medium-high heat. Add walnuts and cook, stirring occasionally, until golden and fragrant, 3-5 minutes. Transfer to a plate. Heat a drizzle of olive oil in same pan over medium-high heat. Add asparagus, salt, and pepper. Cook, stirring, until browned and just tender, 4-6 minutes. Turn off heat; transfer to a plate. Wash out pan.

Add cubed butter to same pan over medium heat. Cook, stirring, until melted and foamy. Continue stirring until foaming subsides and butter is flecked with amber bits, 2-3 minutes. Immediately remove from heat and stir in garlic, scallion whites, juice from half the lemon, and chili flakes to taste. Stir until fragrant, 30-60 seconds.

Return same pan to medium heat and stir in rigatoni, stock concentrate, and asparagus. Season with salt and pepper. Add half the Parmesan (save the rest for serving) and a splash of reserved pasta cooking water. Cook, stirring and adding splashes of pasta cooking water as necessary, until smooth and creamy, 2-3 minutes. TIP: If cheese clumps, continue stirring and adding splashes of pasta cooking water until smooth.

Taste and season rigatoni with salt and pepper; divide between bowls. Dollop with lemony ricotta and sprinkle with scallion greens, toasted walnuts, remaining Parmesan, and a pinch of chili flakes if desired. Drizzle with olive oil. Cut remaining lemon into wedges and serve on the side.