Set a large skillet over medium-high heat and add 1 tbsp of olive oil. Once shimmering, add the asparagus along with a generous pinch of salt. Cook, tossing frequently, until the spears turn bright green and begin to tenderise, about 2 minutes.
Add the lemon zest, garlic and chili flakes (if using) to the skillet. Toss until combined with the asparagus and cook until fragrant, about 30 seconds to 1 minute.
Pour over the lemon juice and cook for another 30 seconds to 1 minute or until the lemon juice has reduced and has coated the asparagus evenly. Serve.