2 medium heads cauliflower (about 3 pounds total), cut into bite-size florets (about 12 cups)
1/2 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper, divided
8 strips bacon (about 8 ounces)
2 tablespoons all-purpose flour
1 1/2 cups whole milk
1 1/2 cups grated sharp white or yellow cheddar cheese (about 6 ounces)
3 medium scallions, thinly sliced, divided
3/4 cup grated Parmesan cheese (about 1 1/2 ounces)