In a large frying pan over medium heat, cook the chopped bacon until fully cooked, stirring occasionally. This will take about 5-7 minutes.
Remove the cooked bacon with a slotted spoon and pat dry with paper towels, leaving the bacon grease in the pan.
Add the sliced onion to the skillet and cook for 4-5 minutes until translucent. Add minced garlic, stir, and cook for an additional 1-2 minutes. Transfer the cooked onions and garlic to a slow cooker using a slotted spoon.
To the slow cooker, add chicken broth, water, salt, pepper, red pepper flakes, and half of the cooked bacon. Stir well to combine.
Add the green beans to the slow cooker, ensuring they are mostly covered with liquid. If needed, add more stock or water until the beans are mostly covered.
Cook on high for 4-5 hours or low for 6-8 hours until the beans reach the desired tenderness, stirring occasionally. Taste the beans at the 4-hour mark if cooking on high.
Once cooked, drain the liquid from the slow cooker. Discard the cooked bacon and onions if desired. Transfer the beans to a large bowl and toss with the remaining crispy bacon.
Optional: Garnish with grated or shaved Parmesan cheese before serving for extra flavor.
Notes
For a vegetarian option, omit the bacon and use vegetable broth instead of chicken broth.