Preheat oven 475
Bring a gallon of water to a boil in an 8-quart saucepan.
Add the beans and blanch for 5 minutes.
Drain in a colander and immediately plunge the beans into a large bowl of ice water to stop the cooking.
Drain/dry and set aside.
Melt the butter in a 12-inch cast iron skillet on medium-high heat.
Add the onions, 1 teaspoon salt and pepper and cook.
Stir occasionally.
Add the garlic and cook 2 more mins.
Sprinkle the flour over the mixture and stir to combine.
Cook 1 minute.
Add the broth and simmer 1 minute.
Lower the heat and slowly add half & half.
Cook until it thickens about 6 -8 mins.
Remove from the heat and stir in 1/4 of the onions and all of the green beans.
Top with the rest of the onions.
Bake until bubbly, approximately 12-15 minutes.