Bring a large pot of salted water to a boil. Add the cauliflower florets and cook until just tender, about 7-8 minutes. Drain the cauliflower well in a colander.
Over medium heat, melt 2 Tbl butter in a medium-sized saucepot. Whisk in the flour, then cook for about 3 minutes, whisking frequently. Add the milk in a slow stream, just a little at a time, whisking constantly. Reduce the heat to keep the sauce at a bare simmer. Cook for about 10 minutes, whisking frequently. Turn off the heat, then add 6 oz cheddar cheese and whisk until the cheese is melted. Add the sliced scallion greens, then season with a good pinch of salt and pepper. Stir to combine. Combine the cauliflower and sauce in a large mixing bowl and stir to coat thoroughly. Pour the mixture into the prepared casserole dish.
Make the bread crumb crust: Add the bread crumbs to a medium mixing bowl. Melt the remaining 2 Tbl butter and pour over the bread crumbs. Stir with a rubber spatula to combine. Add the remaining 2 oz grated cheddar. Season with generous pinches of salt and pepper. Stir to combine.
Sprinkle the crumbs evenly over the cauliflower mixture. Bake, uncovered, until the topping is golden brown, about 10 minutes.
