Heat the olive oil in a nonstick pan on medium heat. Add the garlic and sauté until fragrant (about 30 seconds - 1 minute). Then add the shrimp and lemon zest, season with salt and pepper, and sauté for about 4 minutes, until the shrimp is no longer translucent. Remove the shrimp and set them aside.
Pour in the broth; add the lemon juice and red pepper flakes (if using) and bring to a simmer for 5 minutes. Add in the cornstarch and water mixture and simmer until the sauce becomes a thick and smooth consistency. Stir in the butter.
Toss in the shrimp, parmesan, and parsley; serve with crusty bread.