Heat oven to 200C/180C fan/gas 6. Place the bread slices on a large baking tray in a single layer and drizzle over 1 tbsp extra virgin olive oil. Bake for 10-12 mins or until dry, taking care not to let them burn or become too hard.
Heat the remaining oil in a medium non-stick frying pan and cook the anchovies, garlic and onion for 3-4 mins over a low heat until well softened, stirring until the anchovies break up. Add the tomatoes, purée, capers and chilli flakes, and cook over a low heat for 10-15 mins, stirring regularly, until the sauce is thick. Leave to cool for 15 mins.
Once cool, spread a little tomato sauce on each piece of bread and top with half an olive. Sprinkle with the Parmesan, then cover and chill for up to 24 hrs until ready to reheat.
Heat oven to 200C/180C fan/gas 6. Put the toasts on a parchment-lined baking tray (or two) and bake for 8 mins or until hot throughout. To serve, garnish with basil and a drizzle of extra virgin olive oil.