Step 1: Cook the Pasta. Bring a large pot of generously salted water to a rolling boil. Add spaghetti and cook according to package directions until al dente. Before draining, reserve about 1 cup of the starchy pasta water. Drain the pasta and set aside.
Step 2: Sauté Aromatics and Asparagus. In a large skillet or Dutch oven, heat 1 tablespoon of olive oil over medium heat. Add minced garlic and sauté for 1 minute until fragrant, being careful not to burn it.
Step 3: Add the asparagus to the skillet. Sauté for 3-5 minutes, stirring occasionally, until the asparagus is bright green and crisp-tender. Season with 1/2 teaspoon salt and 1/4 teaspoon black pepper.
Step 4: Combine and Form the Sauce. Add the drained pasta directly to the skillet with the asparagus and garlic. Pour in the fresh lemon juice and about 1/4 cup of the reserved pasta water. Add the optional butter.
Step 5: Emulsify and Season. Toss everything together vigorously with tongs, adding more pasta water a tablespoon at a time if the sauce seems too thick, until a light, creamy sauce forms and coats the pasta beautifully.
Step 6: Finish and Serve. Stir in 1/2 cup of grated Parmesan cheese and chopped fresh parsley. Taste and adjust seasoning as needed (more salt, pepper, or lemon juice). Serve immediately, garnished with extra Parmesan and a drizzle of olive oil.