Bring a large pot (at least 6–8 quarts) of water to a rolling boil and salt it generously with 1.5 tablespoons kosher salt. Salting the water seasons the pasta from the inside out — it should taste like seawater. While the water heats, trim woody ends from the asparagus, slice into 1–1.5" pieces and set aside. Pat the scallops very dry with paper towels (multiple passes) and, if they have side-muscle tabs, remove them. Dry scallops will sear instead of steam, giving a deep, caramelized crust.

Cook the gluten-free linguine per package instructions, starting it once the water is boiling. Set a timer for the package's recommended time. About 2–3 minutes before the pasta is al dente, add the asparagus pieces to the pot so they cook just until bright green and tender-crisp. Reserve 1½ cups (≈360 ml) of starchy pasta water before you drain. Reserving pasta water is crucial — the starch helps the pesto emulsify into a silky sauce that will cling to the noodles.

While the pasta cooks, season the scallops lightly with salt and a few turns of freshly cracked pepper. Heat a large stainless-steel or cast-iron skillet over medium-high heat until shimmering — this ensures the pan is hot enough to produce a quick Maillard crust. Add 1–2 tablespoons of the olive oil and swirl to coat.

Add the scallops to the hot pan spaced at least 1 cm apart (do not overcrowd; sear in batches if needed). Leave them undisturbed for 1¾–2¼ minutes depending on size, until a deep golden-brown crust forms. Flip and cook the other side about 1–1½ minutes. The short, high-heat sear locks in sweet scallop juices and gives contrast to the soft pasta. Transfer seared scallops to a plate and tent loosely with foil to rest for a couple of minutes (this relaxes carryover cooking).

Wipe the pan lightly if there’s excessive residue, then return it to medium heat and add the remaining olive oil (about 2 tbsp). Add the minced garlic and gently cook 20–30 seconds until fragrant — you want the garlic to bloom and soften without browning, which would make it bitter. Add the pesto to the pan and stir to warm it through; heating the pesto briefly brightens the herbs and loosens the oil.

Add ½ cup (120 ml) of the reserved pasta water to the pesto and whisk or stir vigorously to emulsify into a smooth sauce. Add more water (a tablespoon at a time) until you reach a silky, slightly loose consistency — it should coat the back of a spoon. The starch in the water acts as a binder so the pesto clings to the linguine rather than pooling on the plate.

Add the drained linguine and asparagus to the pan with the pesto. Toss vigorously for 1–2 minutes over low–medium heat to marry the flavors and finish cooking the pasta to al dente. Twirling the pasta in the sauce for a minute integrates the starches and oil and gives the dish a glossy finish. Taste and adjust seasoning with salt, freshly cracked pepper, and a pinch of red pepper flakes if using.

Divide the pesto-coated linguine and asparagus among four warmed plates or a large serving platter. Top each portion with 6–8 seared scallops. Finish each plate with a generous scattering of lemon zest, a final crack of black pepper and a small drizzle of olive oil (or a light pat of butter if you want extra silkiness). Serve immediately with lemon wedges on the side. The lemon zest lifts the herbal pesto and brightens the seafood flavor right before eating.

To check scallop doneness (if unsure): they should be opaque through the middle and springy to the touch, not rubbery. If you have an instant-read thermometer, aim for about 115–125°F (46–52°C) out of the pan — they will carryover cook to perfect doneness while resting.