Place a large saucepan or soup pot over a medium heat, and add the olive oil.
Add the onion to the pot, and saute for 2 minutes or until golden brown.
Add the bacon and cook for a further 5 minutes, or until the fat starts to release and the bacon is browned. I like scrape the onion to one side in the pot as I find the bacon browns better this way. You can also remove the onion from the pot whilst browning the bacon if you prefer.
Add the carrot and celery to the onion mixture and saute for 3 - 5 minutes, or until softened.
Add the garlic and cook for 1 minute.
Add the crushed tomatoes, red lentils, bay leaf and thyme sprig
Pour the chicken stock into the pot, ensuring the ingredients are well covered.
Bring the soup to the boil, then reduce the heat to a simmer.
Cook for 30 minutes, or until the lentils have softened and are starting to collapse.
Check the seasoning, and add salt & pepper if required. Some bacon is quite salty and you may not need to add additional salt.
Serve with loads of crusty bread.