Sprinkle the shrimp with a scant 1/4 tsp. salt and a few generous grinds of black pepper. Using a peeler, gently shave the zest in strips from the lemon, taking care not to get any of the bitter white pith. Squeeze the lemon to get 1 Tbs. juice

Put a 12-inch skillet (not nonstick) over medium-high heat for 1-1/2 minutes. Add 2 Tbs. of the oil and once it's shimmering hot, add the shrimp in a single layer. Cook undisturbed until the shrimp browns nicely, about 2 minutes. Flip the shrimp and brown the second side, about 1-1/2 minutes. Transfer to a large plate. The shrimp should be a little undercooked.

Reduce the heat to medium, add the remaining 4 Tbs. oil and the garlic and cook, tossing, until the garlic starts to sizzle steadily, about 30 seconds. Add the asparagus, lemon zest, and red pepper flakes, sprinkle with 1/2 tsp. salt and cook, tossing often, until the garlic is golden brown and the asparagus looks blistery in places, 2 to 3 minutes. Add the chicken broth, cover, with the lid ajar, and cook until the asparagus is just tender, 1 to 2 minutes.

In a small dish, whisk together the cornstarch with 1 Tbs. water, stir into the asparagus mixture, and bring to a boil. Stir in the shrimp, reduce the heat to low, and cook, tossing, until the shrimp is opaque throughout (cut one in half to check), 1 to 2 minutes. Stir in the 1 Tbs. lemon juice and then add salt, pepper, and additional lemon juice to taste. Serve immediately.