In a medium bowl, whisk together the olive oil, orange juice, lemon juice, garlic, onion, 2 teaspoons of the parsley, and pinch of red pepper flakes.
Pour the mixture into a large skillet set over medium heat. Bring to a simmer and cook until reduced by half, about 5 to 8 minutes.
Add the shrimp, season with kosher salt and freshly ground black pepper, cover, and cook until they turn pink, about 5 minutes.
Top with the remaining parsley and serve with orange and lemon slices on the side.