Cook the bacon in a skillet over medium heat until brown and crisp. Use a slotted spoon to remove the bacon from the grease in the pan and let it drain on a paper towel lined plate while you prepare the rest of the dish.
Preheat the oven to 350ºF. Remove the leaves and stem from the cauliflower, then cut it into small florets. Add the cauliflower florets to a pot and cover with water. Place a lid on the pot and bring it to a boil over high heat. Let the cauliflower boil for about 5 minutes, or until it is fork tender. Drain the cauliflower well in a colander.
While the cauliflower is draining, prepare the cheese sauce. Add the butter and flour to a sauce pot. Melt the butter over medium heat, then continue to cook and stir the flour and butter for about two minutes more.
Whisk the milk into the flour and butter until it is fully dissolved, and no lumps remain. Continue to heat and stir over medium until the milk reaches a gentle simmer, at which point it will thicken to the consistency of heavy cream.
Season the white sauce with garlic powder, salt, and pepper. Remove the sauce from the heat, then begin adding the shredded vintage cheddar, one handful at a time, making sure it has fully melted in before adding the next handful. If the sauce becomes too cold to melt the cheese, place it over low heat only until all the cheese has melted into the sauce.
Place the boiled and drained cauliflower in a 2-quart casserole dish. Pour the cheese sauce over the cauliflower, making sure it is fully covered. Top with the remaining 2 oz. shredded mild cheddar and the French Fried onions.
Bake the cauliflower for only ten minutes, then switch the oven from bake to broil and broil for 3-5 minutes, or just until the top is slightly browned (broiling is optional). Do not over bake the casserole, or the cheese sauce will split.
Remove the casserole from the oven and top with the cooked bacon and sliced green onions. Serve hot.