How to Make It
Preheat oven to 450°F. Score bottom of tomatoes with a small 1/4-inch-deep "x"; cut tomatoes in half. Toss together tomatoes garlic, red pepper, 2 tablespoons of the oil, 1 teaspoon of the salt, and 1/2 teaspoon of the black pepper in a medium bowl. Place tomatoes, cut side down, on a wire rack set in a rimmed baking sheet. Bake in preheated oven until skins loosen, about 12 minutes. Cool 10 minutes. Gently peel tomatoes; discard skins. Decrease oven temperature to 325°F, and return tomatoes to oven. Bake at 325°F until slightly dehydrated and just beginning to brown, about 1 1/2 hours. Cool completely, about 1 hour.
Process tomatoes, vinegar, oregano, 1 teaspoon of the salt, and remaining 1/2 teaspoon pepper in a food processor until smooth, about 15 seconds. Set aside.
Preheat oven to 550°F. Heat a 12-inch cast iron skillet on stovetop over medium-high. Add sausage; cook, stirring occasionally to break into large pieces, until browned, 6 to 8 minutes. Transfer to a plate lined with paper towels. Wipe skillet clean, and return to medium-high. Sprinkle skillet lightly with cornmeal, and carefully place 1 dough round in skillet, pressing 1/2-inch up sides. Brush with 1 tablespoon of the oil; sprinkle with 1/4 teaspoon salt. Cook dough over medium-high until it just begins to bubble, about 1 to 2 minutes. Spread 1/4 cup tomato sauce over crust. Top with half each of mozzarella, sausage, pepperoni, and onion.
Bake in preheated oven until golden brown and starting to char, 10 to 12 minutes. Let pizza stand 5 minutes; transfer to a cutting board. Repeat with cornmeal and remaining dough round, oil, and salt; spread with 1/4 cup tomato sauce. Top with remaining mozzarella, sausage, pepperoni, and onion. Garnish with basil, if desired; serve with remaining tomato sauce for dipping.