Add the butter, olive oil and salt into a large saucepan and place over a medium heat until melted.
Whilst the butter is melting, peel the onions and finely slice them.
Peel the apple and chop it into small chunks.
Add the onion and apple into the saucepan and toss to make sure everything is covered with the butter and olive oil.
Pop a lid onto the saucepan and leave to cook for 30-40 minutes, stirring every 5 minutes, until the apple and onion is just starting to turn golden brown.
Preheat the oven to 430F/220C half way through the caramelisation process to give it enough time to warm up.
Sprinkle the grated cheese all over the pizza base and top with the caramelised onions and apple.
Add the bacon into the same saucepan and cook for 5-10 minutes, until soft.
Place the cooked bacon onto some kitchen paper to soak up the extra fat then sprinkle it evenly across the pizza.
Brush the outer crust of the pizza dough with olive oil then bake in the centre of the oven for around 20 minutes until the crust is golden and the cheese has melted.