In a medium skillet, heat olive oil over medium-high heat. Add the onion and balsamic vinegar and cook, stirring occasionally, for 15 minutes; season with salt and pepper.
Meanwhile, preheat a grill pan over medium-high heat. Rub the steaks with olive oil, salt and pepper. Cook for 6 minutes on each side for medium-rare; remove and tent with foil.
Grill the bread, cut side down, until toasted, about 4 minutes. Slice each steak in half crosswise; place one half on each baguette bottom (trim to fit, if necessary). Top each with a cheese slice, some onions and a baguette top.