Preheat oven to 350°. Prepare an ice bath. In a large pot of boiling water, add green beans and cook until bright green, about 6 minutes. With a slotted spoon or tongs, quickly transfer green beans to ice bath to cool, then drain and transfer to a large bowl.
In a large, high-sided skillet over medium heat, heat oil. Cook bacon until crisp, 8 minutes. Remove bacon from skillet and drain on a paper towel lined plate.
Over medium heat, whisk in flour and cook until lightly golden and no longer raw, about 2 minutes.
Add milk, Worcestershire, and garlic powder and bring up to a simmer, stirring occasionally. Let mixture thicken slightly, about 2 minutes, then whisk in 2 1/2 cups cheddar until completely melted. Season with salt and pepper and remove from heat.
Combine green beans and cheese sauce in a medium baking dish. Top with remaining cheddar and bake until warmed through and bubbling around the edges, about 20 minutes.
Top with bacon and fried onions and bake 5 minutes more.
Garnish with chives before serving.
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