In a skillet over medium heat, cook bacon until the fat renders and the pieces start to crisp, about 5 minutes.
Add the onion and garlic to the skillet with the bacon and cook until soft and golden, about 5 minutes.
Move the bacon, onions, garlic, and drippings to the slow cooker. Layer the whole haricot verts on top. Pour in the chicken broth, brown sugar, 1 tsp apple cider vinegar, salt and black pepper. Toss gently to coat.
Slow cook for:Fresh whole haricot verts: HIGH 2.5–3 hours or LOW 4–5 hours.Frozen whole haricot verts: HIGH 2 hours or LOW 3–4 hours.
Stir gently halfway through. Beans should be tender but still hold their shape.
Taste and adjust salt and pepper. Stir in butter and the remaining 1 tsp apple cider vinegar for brightness. Serve with the bacon and onion mixture spooned over the beans.
*Fresh French green beans (haricot verts) work best in the slow cooker because they are usually stringless and cook evenly. If using regular green beans, trimming and stringing is essential to avoid tough strings.**We use Better Than Bouillon and base prices on it for chicken broth.