Step 1

In a large bowl, toss the asparagus with 2 teaspoons of oil and ¼ teaspoon each salt and pepper. In a 12-inch cast-iron skillet over medium-high, heat the remaining 1 teaspoon oil, spreading it with a silicone spatula or heatproof brush to cover the surface, until barely smoking. Add half the asparagus, distributing it in an even layer, and cook without stirring until well charred on the bottom, 5 to 6 minutes. Stir, redistribute in an even layer, then continue to cook until tender-crisp, about 2 minutes. Transfer to a medium bowl and partially cover.

Step 2

Return the skillet to medium-high and heat until barely smoking. Cook the remaining asparagus in the same way, then transfer to the bowl with the first batch. To the asparagus, add the butter and toss until melted. Add the lemon juice and tarragon, then toss again. Taste and season with salt and pepper, then transfer to a serving platter.