Marinate steak in 2 tablespoons vinegar and chopped garlic 15 minutes at room temperature. In a separate bowl, combine onion and 1 tablespoon vinegar.
Heat wok or large skillet over medium-high heat; add oil. Remove steak from marinade, reserving marinade, and cook steak 2 minutes. Transfer with tongs to plate; cover.
Add onion to wok, and cook 3 minutes or until slightly softened. Transfer to plate with a slotted spoon; cover.
Add tomatoes to wok; cook 2 minutes. Add reserved marinade; bring to a boil. Add radicchio and remaining 1 tablespoon vinegar; remove from heat.
Rub toasts with cut sides of remaining garlic clove until fragrant. Top each with steak and vegetables. Garnish with rosemary.
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