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This is a serious sandwich that'll take you to summer a bit early. Flank steak gets marinated in an herb and garlic-based Argentinean-style chimichurri flavored with arugula, which serves double duty as a spread for the grilled Texas toast later on. Thick slices of tomato complete this simple but imposing sandwich. Make sure to set aside some chimichurri from the start to mix with your mayo for the sandwich.