Bring a large pot of salted water to a boil. Add green beans and boil for 8 minutes, or until just tender. Drain into a colander, then plunge the colander with the beans into a bowl of ice water. Drain again.
Meanwhile, in a large frying pan over medium heat, cook chopped bacon until crispy, about 10 minutes. Remove with slotted spoon to a paper towel lined plate. Remove all but 1 tablespoon of bacon grease from frying pan.
Carefully add onions to bacon grease and cook over medium heat, stirring, for 5 minutes or until translucent. Add garlic and continue to cook, stirring, for 1 minute or until fragrant.
Add red pepper flakes and green beans, stirring to combine (tongs work great here, too!).
Sprinkle with bacon immediately before serving. Serve with lemon wedge, if desired.
Video
Notes
I like to use center cut bacon because it has more meat and less fat.
If you would like to use frozen green beans instead of fresh, cook beans as directed on bag, cool in ice bath, and proceed with step 2, as directed.
For canned green beans, begin with step 2, drain beans, and add them to the bacon mixture to heat.