Heat a heavy frying pan over a high heat until it is just beginning to smoke. Sear the red onion and leek cut-side down and drizzle with the extra virgin olive oil. Reduce the heat to medium-high and cook for 10-12 minutes, turning the vegetables until evenly charred on all sides. Remove from the heat and allow to cool slightly, then chop the into half inch pieces. Return the same pan to a medium heat and fry the bacon bits until lightly browned. Add the celery and cook for a further 5 minutes, then throw in the garlic and cook again for 2 minutes. Reduce the heat, add the chopped leeks and red onion and cook for another 5 minutes, then turn off the heat. In a separate pan, cook the pasta following the instructions on the packet. A minute or two before the pasta is ready, return the pan with the vegetables to a medium heat and add the cream. When the pasta is cooked, drain and add to the vegetables, stirring gently. Finally, add the lemon juice and zest, season, then serve and enjoy.