Core your tomatoes and chop them into uniform(ish) pieces.
Roll the basil the long way into a cylinder and make teeny little slices to make thin ribbons.
Chop the green onion, dice the oregano, and smoosh the roasted garlic to break up the cloves. Add the balsamic. Mix gently. Top your bread/crostini.
Notes
If you want to use fresh garlic instead of roasted, reduce to three cloves.To make ahead, mix all ingredients except the balsamic. The balsamic can turn your tomatoes an unappetizing color if left too long.You can store the tomato mixture at room temperature for four hours. After that, I would move it to the fridge.