Bring a large saucepan of water to a boil. Cook pasta for 6 minutes. Add asparagus and continue cooking, stirring occasionally, until the pasta and asparagus are just tender, about 3 minutes more. Drain and return to the pot.
2
Meanwhile, whisk milk, mustard, flour, salt and pepper in a medium bowl. Heat oil in a small saucepan over medium-high heat. Add garlic and cook, stirring constantly, until fragrant and lightly browned, 30 seconds to 1 minute. Whisk in the milk mixture. Bring to a simmer, stirring constantly, and cook until thickened, 1 to 2 minutes. Stir in tarragon, lemon zest and juice.
3
Stir the sauce into the cooked pasta and place over medium-high heat. Cook, stirring constantly, until the sauce is thick, creamy and coats the pasta. Stir 1/4 cup Parmesan into the pasta until combined. Serve the pasta topped with the remaining 1/4 cup Parmesan.
