Flank Steak: Preheat oven to 450°F. Toast caraway seeds in small nonstick skillet over medium heat until seeds darken and begin to smoke, stirring often, about 5 minutes. Add olive oil and garlic cloves to caraway seeds in skillet. Cover; remove from heat. Let stand 1 minute. Pour caraway mixture into a food processor. Add chili paste, tomato paste, cumin, and chili powder and blend until garlic cloves are pureed. Season the harissa rub to taste with salt.
Season beef all over with salt and pepper; place beef, fat side down, on rack on sheet pan. Spread with half of harissa. Turn beef over; spread remaining harissa over top and sides. Roast the flank steak until thermometer inserted into center registers 125°F to 130°F for medium-rare, about 15-20 minutes.