Preheat the oven to 200 degrees Celsius (400 degrees Fahrenheit)
In a bowl toss the halved cherry tomatoes with the olive oil, garlic, oregano, salt and pepper. Lay the tomatoes out evenly on a baking sheet or in a cast iron skillet, reserving the excess oil in the bowl to use later.
Bake the tomatoes in the oven for 25 minutes, flipping them half way through cooking to ensure they don't burn. The tomatoes will cook quicker if you drain any excess liquid from the pan into the reserved oil bowl half way through cooking. Remove the tomatoes from the oven once they have begun to shrivel and lose their moisture. Remove from the pan/skillet and set aside
While the tomatoes are cooking prepare the mushrooms by cleaning them, removing the stem and using a spoon to scoop out the gills. Place the cleaned mushrooms in the skillet/pan used to cook the tomatoes.
Brush the mushrooms all over (inside and out) with the reserved oil from the tomatoes. Fill each mushroom with a spoonful of pesto and then top with 3 to 4 roasted tomatoes,
Bake the mushrooms in the oven for 20 minutes until they have completely softened. Top with the chopped basil and a drizzle of balsamic before serving.