Step 1: Bring medium pot of salted water to a boil. Wash & 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.

Step 2: Once water is boiling, add pasta to pot. Cook until al dente. Reserve 1 cup of 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.

Step 3: 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 plate. Wash out pan.

Step 4: 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.

Step 5: Return same pan to medium heat and stir in pasta, 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.

Step 6: Taste and season pasta with salt & 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.