Heat oven to 350°F. Grease a 9x13-inch baking dish.
Cook bacon in a large skillet over medium heat until crisp, 8–10 minutes. Transfer to paper towels, reserving 2 tbsp drippings.
Sauté onion in the bacon drippings until softened, 3–4 minutes. Stir in garlic for 30 seconds. Remove from heat.
Whisk melted butter, brown sugar, soy sauce, Worcestershire, broth, vinegar, pepper, red pepper flakes, and salt in a bowl.
Toss green beans, cooked bacon, onions, and garlic in the prepared baking dish.
Pour the sauce over the mixture and stir to coat evenly.
Cover with foil and bake for 25 minutes.
Uncover, stir, and bake 10–15 minutes more, until bubbling and beans are tender.
Rest 5 minutes, then taste and adjust seasoning. Serve warm.