Heat the olive oil in a large saucepan over medium heat. Add the onions; cook and stir until they turn soft and begin to turn golden brown, about 10 minutes. Stir in the brown sugar, and continue cooking and stirring until the onions are deep brown, about 10 minutes more. Pour in the balsamic vinegar, and simmer until the vinegar has reduced and become syrupy.
Meanwhile, place the bacon in a large, deep frying pan, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon rashers on a kitchen paper-lined plate then dice and set aside.
Preheat an oven to 190 C / Gas 5.
To assemble the pizzas, spread each pitta with 8 tablespoons of pumpkin houmous and place onto a baking tray. Sprinkle with about 4 tablespoons of the caramelised onions. Top with some of the apple and bacon. Finally, sprinkle with 8 tablespoons of fontina cheese.
Bake in the preheated oven until the pizza is hot and the cheese is bubbly and golden brown, about 10 minutes.
For the onions, cut them from one root end to the other and then slice them in little strips between the roots. They will be narrower on the ends and this keeps them from disintegrating during the caramelising process.