Bring a large pot of water to a boil, add 1 Tbsp salt and 1 Tbsp olive oil, and cook pasta according to package instructions until Al'dente, or desired doneness (meanwhile, continue with recipe). Drain pasta, return to pot, and cover to keep warm.: Listen for the vigorous roar of the water before adding the spaghetti , the bubbles should be large and steady which indicates it is hot enough so the strands will not stick together. The starchy scent that rises as the pasta cooks is normal and signals that a little of that starch will help bind the sauce later. I salt the water liberally so the spaghetti is seasoned from within, and adding a touch of olive oil can help prevent foaming and sticking while you manage other steps. When you test for doneness, bite into a strand to check for a firm center, and aim for that slight chew, because it will continue to soften slightly when tossed with the sauce. A common pitfall is overcrowding the pot; if you do that, strands clump and cook unevenly.
Place a large, deep pan over medium/high heat and add 1 Tbsp olive oil and 1 Tbsp butter. Add asparagus, season lightly with 1/4 tsp salt and 1/8 tsp pepper, and cook uncovered for 5 minutes or until crisp-tender, stirring occasionally. Remove from pan.: Once the fat is shimmering and the butter has melted with a gentle foam, add the asparagus ; you should hear a lively sizzle which means the surface will caramelize slightly. Stirring occasionally lets each piece get a flash of color without steaming, and the aroma will shift to a green, toasty note as the sugars begin to brown. Cook until the spears are verdant and yield to gentle pressure from a fork, this crisp tenderness keeps them lively against the soft spaghetti . If you cover the pan or cook at too low a temperature, the asparagus will become limp and lose its bright flavor.
In the same hot skillet, add 2 Tbsp oil and 2 Tbsp butter. Once the butter is melted, add minced garlic and red pepper flakes if using, and sauté stirring frequently for 1 minute. Add shrimp in a single layer and season with 1 tsp salt and 1/4 tsp black pepper. Sauté until shrimp are pink and not translucent (about 1 minute per side). Be careful not to overcook or the shrimp will be tough.: As the second portion of fat warms, the smell of butter and olive oil combining becomes nutty and inviting, which is the perfect foundation for the pungent sweetness of the garlic . Sautéing the garlic for only about a minute releases its fragrance without letting it brown and turn bitter; you will notice its aroma bloom and then stabilize. Lay the shrimp in one layer so each piece makes contact with the hot pan, you should hear a crisp sizzle as they hit the surface; that sear locks in juices and yields pleasant caramelized edges. Watch closely as the shrimp transition from translucent to opaque and pink; they will curl and firm up, and at the right moment they will spring slightly when nudged. Overcooking is the classic mistake here, producing a rubbery texture instead of tender bites.
Add lemon zest, juice, and parsley, and stir to combine. Turn off the heat and return the pasta and asparagus to the pan. Toss everything to combine and serve with freshly grated parmesan.: The moment you add the grated lemon zest and fresh lemon juice the pan brightens, releasing citrus oils that perfume the sauce and cut through the richness of the butter . The aroma becomes lifted and clean, and the chopped parsley introduces a fresh, green scent that rounds the profile. Turning off the heat before adding the spaghetti and asparagus prevents the shrimp from overcooking while the strands absorb the sauce. As you toss, you should see the glossy coating cling to the spaghetti , and steam will rise carrying a balanced mix of lemon, garlic, and butter. Finish by grating freshly grated parmesan over each portion so it melts slightly into the warm noodles for a savory, rounded finish. If you toss while the pan is too hot, the cheese can clump rather than melting evenly, so moderate the heat carefully.