Heat the oven to 220c.
Briefly sauté the onions and garlic for five minutes in some olive oil with a pinch of salt and pepper.
Add the tomatoes and balsamic vinegar and cook for one minute more then add half the basil, chopped.
Top the pizza bases with this mixture and cook in the oven for 15 minutes.
Scatter with the remaining basil and a good drizzle of olive oil and finish with black pepper, chilli flakes and a good scattering of grated parmesan.