Heat 2 tablespoons oil in a large, broiler-safe skillet over medium heat; add 5 cups cauliflower and ½ cup onion; cook, stirring occasionally, until the cauliflower is lightly browned and the onion is softened, about 5 minutes.
Add 1 tablespoon tomato paste, the minced garlic, 1 teaspoon Italian seasoning, ½ teaspoon salt and ¼ teaspoon crushed red pepper; cook, stirring constantly, until fragrant, about 1 minute.
Stir in 1 (15-ounce) can tomato sauce, ½ cup heavy cream and ¼ cup vodka; reduce heat to maintain a gentle simmer. Cook, stirring occasionally, until the cauliflower is fork-tender and the sauce is thickened, 8 to 10 minutes.
Meanwhile, preheat oven to broil with rack in top third position.
Sprinkle ¾ cup mozzarella over the cauliflower mixture. Broil until the cheese is melted and lightly browned, about 5 minutes. Garnish with basil, if desired. Serve with toasted baguette.
