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Preheat the oven to 375°. In a large skillet, heat 1 tablespoon of the olive oil until shimmering. Add half of a garlic clove and cook over high heat until golden brown; using a slotted spoon, discard the garlic. Spread half of the squid evenly in the skillet and season with salt. Cook without stirring for 1 minute, then transfer the squid to a large bowl. Repeat with 1 tablespoon of the olive oil and the remaining garlic clove half and squid.
Add the remaining 1 tablespoon of olive oil, the leeks and crushed red pepper to the skillet. Cook over low heat, stirring occasionally, until the leeks are softened, about 8 minutes. Stir in the flour and cook for 1 minute. Add the tomato and wine and bring to a simmer, stirring well to scrape up any browned bits on the bottom of the skillet. Return the squid and its juices to the skillet and simmer, stirring occasionally, until the stew has thickened and the squid is very tender, about 50 minutes. Season with salt and black pepper.
Meanwhile, arrange the bread slices on a large rimmed baking sheet. Brush the slices with olive oil and bake for 8 minutes, or until golden brown. In a small bowl, mix the mayonnaise with the pounded garlic and season with lemon juice and salt.
Mound the squid on the crostini. Top with aioli and the arugula and serve.