In a small bowl, whisk together olive oil, balsamic vinegar, tamari, and black pepper. Brush portobello mushrooms generously with the marinade and place in a baking dish.Cover tightly and let the marinade absorb. Preheat the oven to 350°F.
When the oven is hot, bake for 15-20 minutes, or until mushrooms are tender and juicy. Remove from oven and allow to cool.
Slice the ciabatta loaf in half lengthwise. Spread goat cheese evenly on both cut sides. Layer the bottom half with tomato slices, portobello mushrooms, and arugula. Top with the remaining bread.
Cut into four sandwiches and serve immediately.