Trim the tough ends from the asparagus and cut into 2-inch pieces, keeping the tips separate for garnish. Mince the garlic, zest the lemon (reserving about 1 teaspoon for garnish), and juice the lemon into a small bowl. Measure out the crème fraîche, parmesan, and pepper. Bring a large pot of salted water to a boil and add the spaghetti, cooking according to package directions until al dente.

While the pasta cooks, melt the butter in a large skillet over medium heat. Add the minced garlic and sauté for about 30 seconds until fragrant, then add the asparagus pieces (not the tips) along with most of the lemon zest. Cook for about 5 minutes, stirring occasionally, until the asparagus is tender-crisp. I like to keep the asparagus slightly undercooked at this stage since it will warm through again when tossed with the pasta.

Add the fresh lemon juice to the asparagus in the skillet and stir to combine. Pour in the crème fraîche and add the black pepper, stirring constantly over medium-low heat until the sauce is smooth and heated through—about 2 minutes. Avoid boiling to keep the sauce silky.

When the pasta is done, reserve about 1/2 cup of the starchy cooking water, then drain the spaghetti. Pour the warm pasta directly into the skillet with the sauce and toss to coat. Add the parmesan cheese and a splash of reserved pasta water, stirring gently until the sauce coats the pasta evenly and reaches a creamy consistency—add more water if needed. I find that the starch in the pasta water helps emulsify the sauce beautifully.

Divide the pasta among serving bowls and top each portion with the reserved asparagus tips. Finish with a sprinkle of additional parmesan, a crack of fresh black pepper, and a light dusting of the reserved lemon zest. Serve immediately while warm.