1. Combine 2 tbsp oil, vinegar, garlic and hot pepper flakes in sealable freezer bag. Set aside 2 tbsp of marinade. Using fork, pierce meat all over; add to marinade in bag. Refrigerate for 8 to 12 hours.
2. Remove steak from bag. Discard used marinade. Grill steak over medium-high heat, about 3 to 4 minutes per side for medium-rare. Let steak rest on carving board for 10 minutes.
3. Meanwhile, pan-fry onion in remaining oil over medium heat in non-stick skillet for 10 minutes or until softened and golden. Toss with reserved marinade.
4. Slice steak thinly on the diagonal. Drape slices of steak on each baguette bottom piece. Top each with warm onions and arugula.