Completely thaw green beans then pat dry with paper towel. If using fresh green beans blanch in boiling water for 3-4 minutes then shock in an ice bath to cool to room temperature then set aside
In a large skillet over medium heat melt butter then whisk in flour and cook for about 2 minutes. Whisk in milk and stir until thickened. Add in salt, pepper, garlic, and beef base. Cook for about 1 minute then fold in green beans, cheddar cheese, and ½ of the bacon. cook for about 4 minutes until green beans are warm.
Preheat oven to 350 degrees F. then transfer the green bean mixture to a casserole dish. Bake for 20 minutes. Top with remaining chopped bacon and crispy fried onions before serving.