Start by adding a little olive oil to a heavy based pan over high heat. While the pan is heating, cube your beef into small chunks and then brown it off in the olive oil. Once the beef has browned, remove it from the pan and set it aside.
Now add the bacon to the pan and cook it for around 5 minutes before adding the onion and allowing it to get a little brown.
Now add the garlic, carrots and celery to the pot and cook for a few minutes. Allow the vegetables to deglaze all the flavor off the bottom of the pan. They will sweat a little and the excess water will dissolve all the bits stuck to the bottom of the pan.
In a jug (not in the pan) stir the Guinness, stock and the sourdough starter together so that the sourdough starter isn't lumpy. Popping it into a batter jug to mix makes it easier to pour in later. Set aside.
Return the beef to the pot, along with the tomato paste, salt and pepper and stir through.
Pour the Guinness mixture over the beef and vegetables.
Add the potatoes, bay leaves and thyme and cover the pan with a lid. Allow the beef and Guinness stew to simmer for around 2 hours on a low heat, stirring occasionally to ensure the bottom doesn't catch.
After 2 hours, check the beef for tenderness, if it's not tender, stir and allow the stew to cook down for another hour.
Once you're happy with the beef, remove the lid and increase the heat slightly. Allow the stew to simmer uncovered for around 30 minutes to allow the sauce to thicken a little.