Cook 12 oz. small pasta (such as ditalini) in a large pot of boiling salted water, stirring occasionally, until al dente. Drain, reserving 1 cup pasta cooking liquid.

Meanwhile, season 12 sea scallops (about 1 lb.), patted dry, with kosher salt. Heat 3 Tbsp. extra-virgin olive oil in a large skillet over high. Cook scallops until well browned underneath, about 3 minutes. Turn scallops over; cook until just cooked through, about 1 minute. Transfer to a plate and arrange browned side up.

Melt 4 Tbsp. unsalted butter in same pan over medium heat. Add 1 lemon, thinly sliced, seeds removed, and ¼ tsp. crushed red pepper flakes and season with salt and freshly ground black pepper. Cook, stirring often, until lemon slices are softened, about 2 minutes. Add 3 Tbsp. fresh lemon juice and 1 Tbsp. fish sauce and cook, stirring constantly and scraping up browned bits stuck to bottom of pan, until mixture is loosened and combined, about 30 seconds.

Gradually stream in ¾ cup pasta cooking liquid, stirring constantly, and cook, still stirring, until homogenous, about 2 minutes. Add remaining 4 Tbsp. unsalted butter; cook, stirring, until combined (it should be loose but glossy). Remove sauce from heat and season with salt.

Add pasta and ½ large head of fennel, cored, shaved with a mandoline or thinly sliced; toss to coat. Divide among shallow bowls and set scallops, browned side up, on top. Scatter fennel fronds over and sprinkle with black pepper and red pepper flakes.