In a frying pan, cook the portobello mushroom caps in 1 tablespoon of olive oil, flipping occasionally on medium heat for 10-12 minutes.
While the mushrooms are cooking, in a medium sized bowl, combine the diced tomatoes, diced red onion, crumbled feta, garlic powder, dried basil, and the remaining tablespoon of olive oil. Stir until well combined.
When the mushrooms are done, top each one evenly with the bruschetta mixture and drizzle with balsamic glaze.