Preheat broiler to high. Cook pasta according to package directions for al dente. Drain and set aside.

Stir together cauliflower, 2 tablespoons of the oil, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper in a 12-inch cast-iron skillet. Cover and cook over medium-high until florets begin to brown, about 5 minutes. Add water; partially cover, and cook, stirring occasionally, until cauliflower is tender, 14 to 16 minutes. Transfer to a medium bowl. Wipe skillet clean.

Melt butter in skillet over medium. Add flour, and cook, whisking constantly, until smooth and lightly browned, about 1 minute. Gradually whisk in milk. Bring mixture to a simmer, and cook, whisking often, until thickened, 6 to 8 minutes. Remove from heat, and whisk in cheeses, hot sauce, and remaining 1 1/2 teaspoons salt and 1/2 teaspoon pepper until smooth. Working in batches, carefully fold in cauliflower and pasta.

Stir together panko, green onions, and remaining 2 tablespoons oil. Sprinkle mixture over mac and cheese. Broil until topping is golden brown, 1 to 3 minutes.