Heat olive oil in large skillet over medium-high heat. Add green beans, stirring often until green beans start to blister, about 3-4 minutes. Lower heat to medium, then add in garlic, lemon juice from half the lemon, and zest from whole lemon. Cook for one more minute.
Remove from the heat, add butter (if using) and stir until melted and the green beans are coated. Top with sea salt before serving warm.
Notes
These green beans can be made 1-2 days in advance. Reheat in the microwave or placing them back in a skillet on medium heat until warmed.