Preheat the oven to 425 degrees F. Lightly coat a baking sheet with cooking spray.
Place the portobello mushrooms in a baking dish.
Whisk together the balsamic vinegar and 1 1/2 tablespoons olive oil, and rub the mixture on the mushrooms.
Sprinkle the garlic over the mushrooms, lay one tomato slice on each mushroom, and roast for 10 minutes.
In a small bowl, stir together the Skellig cheese, breadcrumbs, 1 tablespoon olive oil and salt. Sprinkle the cheese mixture evenly over the mushrooms.
Bake for another 5 minutes, or until the breadcrumbs are light golden brown. Serve.