Heat the butter in a large skillet over medium heat until it fully melts.
Add the onion, salt, and black pepper. Cook for 2 minutes, stirring occasionally.
Add the garlic and cook for 3 more minutes, stirring frequently until the garlic is fragrant and the onion becomes translucent.
Add the lime zest and lime juice. Cook for 2 more minutes, stirring occasionally.
Raise the heat to medium-high and add the shrimp. Cook for 3-4 minutes, flipping the shrimp halfway through, until the shrimp turns pink and is just cooked.
Remove from heat, garnish with cilantro, and serve immediately.