Bring a large cast iron skillet to medium-high heat. Add bacon and move around pan until fully cooked, approx. 5-6 minutes (total cook time will depend how crispy or not you prefer your bacon). 

Remove with slotted spoon and place on a paper-towel lined plate; set aside.

Leave approx. 2 tablespoons of bacon grease in the pan and discard the rest. 

Setting the same skillet to medium heat, add the garlic and move around pan until fragrant. Increase heat to medium-high and add green beans. Let green beans cook approx. 8-10 minutes *or until they have reached your desired firmness* tossing them quite often.  Test along the way as some people prefer a more crisp green bean, while others like them quite soft - taste and continue cooking and moving around the pan until they reach this point. 

Add lemon zest, red pepper flakes (if using) and the bacon. Toss to combine. 

Taste and add kosher salt and/or ground black pepper, as desired.

Serve immediately and enjoy!