In a large bowl, whisk together lemon juice, mustard, oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Set aside.
Bring a large pot of generously salted water to a boil. Cook asparagus spears until fork-tender, about 4 minutes; drain. Add to the bowl, along with 1 thinly sliced shallot. Toss to coat.