Prepare the slow cooker by coating with a light coating of cooking spray.
Place the green beans in the prepared slow cooker and set aside.
In a large skillet, cook the chopped bacon until crispy. Drain off fat and set bacon aside.
In the same skillet, cook the onion until tender. 3-5 minutes. Add garlic and cook for 1 additional minute.
Place the onion and about half the bacon over the green beans. (Reserving half the crispy bacon for serving, later)
Pour in the chicken broth and season with salt, pepper, and brown sugar.
Stir gently to combine, then cover and cook on low for 4–6 hours or high for 2–3 hours, until green beans are tender.
Stir in the butter and remaining bacon just before serving, if using.