Slice the Portobello mushrooms and marinate them in the balsamic vinegar.
Prepare a batch of pesto (unless using store bought). There will likely be extra pesto (Can be used for pasta or frozen).
Make a batch of pizza dough.
Preheat oven to 400 °F.
Cook the Portobello mushrooms in a non-stick pan on medium heat, for about 5 minutes to remove some of their water.
Thinly slice the tomato and red onion.
Roll out the pizza dough and place onto a baking tray covered with a light sprinkle of corn meal. To make the crust edge, simple roll the outer edge of the dough inwards and lightly press down so it holds its shape.
Spread a layer of pesto onto the crust and top with the mushrooms, tomato and onion.
Bake for 12-15 minutes, until the crust is golden brown along the edges and on the bottom.