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