Wipe out pan, add remaining 2 Tbsp. olive oil, and set over medium heat. Cook onion, stirring often, until softened but without taking on any color, about 5 minutes. Add garlic and cook, stirring, until softened, about 3 minutes. Add shrimp stock and reserved shrimp and cook, stirring occasionally, until most of the shrimp have turned pink, about 2 minutes. Remove from heat. (Shrimp will continue cooking in the residual heat.) Mix in butter, cilantro, lemon zest, and red pepper flakes; season with salt. Let cool 20 minutes, then transfer to a medium bowl. Chill at least 1 hour before using.
post-title Shrimp Empanadas