Preheat oven to 400 degrees F and line a baking sheet with parchment paper. In a large bowl, toss all of the olive oil with mushroom caps, tomato slices, salt, and pepper. Spread veggies out on baking sheet and roast for about 40 minutes, flipping mushrooms halfway through. On a plate, arrange Portobello mushroom caps and top with tomatoes, feta, and basil.