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Preheat oven to 450 degrees F.
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To make sauce, set a large saucepan over medium heat. Add oil and then red onion. Sprinkle with sea salt. Stirring often, cook about 7 minutes or until onions are tender and translucent. Add garlic and cook one minute more. Stir in crushed tomatoes and another pinch sea salt. Turn heat up so mixture begins to bubble gently, stir, and then turn heat to medium-low. Simmer, stirring often, 15 - 20 minutes or until mixture thickens. Add sea salt and pepper to taste.
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Meanwhile, prep toppings. Heat 2 tablespoons olive oil in a medium skillet over medium heat. Add leeks and a pinch sea salt. Cook over medium heat just until leeks begin to edge golden, turn heat to low and, stirring occasionally, cook until leeks are translucent and sweet, about 20 minutes.
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Heat 1 tablespoon olive oil in another medium skillet. Add mushroom slices, sprinkle with sea salt, cook 2 minutes, flip, and then cook 2 minutes more. Remove to a plate. Add another tablespoon oil to the same skillet (still over medium heat) and toss in Brussels sprouts along with a pinch sea salt. Cook until Brussels wilt and brown along edges. Remove to another plate.
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Set French bread slices on a rimmed baking sheet. Toast in hot oven 5 minutes, or until edges are golden.
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To assemble pizzas, spread several tablespoons sauce on both slices of bread. Top one slice with mushrooms and caramelized leeks, and the other with Brussels sprouts and scallions. Divide mozzarella and Asiago between the two pizzas. Add black pepper to taste. Slide into oven and bake 7 - 10 minutes, or until cheese is melted and just beginning to brown.
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Slice and serve immediately.