Place prepared green beans in a large skillet (or other wide pan with a lid) and add the water. Cover and quickly bring to a boil. Reduce heat so a simmer is maintained, but not a hard boil. You don't want to boil the water away. Steam them like this for 7 minutes. Drain.
Return the pan of beans to the burner and make a clear spot in the center. Pour in the oil and add the garlic and red pepper flakes. Quickly shake to spread the oil. Salt and pepper the beans to taste. Cook, uncovered for 3 minutes on medium heat.
Taste the beans and cook another minute or two if you wish and adjust the seasoning to your liking. Gently stir in the lemon zest and serve immediately. (Don't add the lemon juice or it will turn your beans gray.)