1/4 cup thinly sliced red onion, separated into rings
1 14-ounce can artichoke hearts, rinsed and coarsely chopped
1 medium tomato, seeded and diced
2 tablespoons balsamic vinegar
1 tablespoon extra-virgin olive oil
1 teaspoon dried oregano
1 16- to 20-inch-long baguette, preferably whole-grain
2 slices provolone cheese, (about 2 ounces), halved
2 cups shredded romaine lettuce
1/4 cup sliced pepperoncini, (optional)