Rub garlic all over steak and let stand at room temperature. In small colander or sieve, toss radishes with 1/8 teaspoon salt. Place colander over bowl, cover, and refrigerate. Into small bowl, grate peel from 1 lime and cut lime into quarters; set aside. Stir sour cream into grated lime peel. Cover bowl and refrigerate.
Preheat oven to 300 degrees. Wrap tortillas in foil, and heat in oven 15 minutes or until warm and pliable.
Heat 12-inch cast iron or other heavy skillet on medium-high heat until hot. Brush garlic off steak and discard. Squeeze juice from lime quarters all over steak, then sprinkle with 1/4 teaspoon salt and 1/4 teaspoon freshly ground pepper to season both sides. Add 1 teaspoon oil to skillet, then add steak. Cook 10 minutes for medium-rare or until desired doneness, turning over once. Transfer steak to cutting board; reduce heat to medium.
Add remaining 1 teaspoon oil to skillet and add peppers and onion. Cook 2 to 3 minutes or until onion browns, stirring occasionally. Add 1/4 cup water and cook 5 minutes longer or until vegetables are tender, stirring occasionally.
Cut steak across the grain into thin slices. Cut remaining lime into wedges. Divide steak and peppers and onions among tortillas; top with lime sour cream. Serve with radish salad and lime wedges.