Heat 1 tbsp olive oil in a large heavy based Le Creuset pan Dutch Oven or similar oven-proof pot, over medium heat. Add the diced onion, carrots, and 1/2 tsp sea salt and cook for 5 minutes to soften onions slightly. Add the garlic and cook for 2 minutes more. Remove the vegetables from the pot and set aside.
In the same pot, add another tablespoon of olive oil over medium/high heat, and add 1/2 the beef. Spread out and brown the beef to seal them. Stir so they don't burn. This will only take a few minutes. Remove to a dish and repeat with the rest of the beef adding more oil if required. Once the beef is browned, add all the beef back to the pan, along with the onion mixture, flour, beef stock, beef cube, worcestershire sauce, tomato sauce, bay leaves and thyme. Stir well to lift any bits that have stuck to the bottom of the pot. Bring everything to the boil,and once boiling, cover with a lid and place the pot in the preheated oven to cook for 45 minutes.
After 45 minutes, remove from the oven and add the potato cubes. Stir to combine and to cover the potatoes. Cover and return to the oven for another hour. Check again after 1 hour and stir mixture. It will probably need another 30 minutes to 1 hour for the beef to be tender and potatoes to soften, so put back in the oven and check again in 30 minutes. Once ready, remove from the oven, discard the bay leaves and thyme sprig, and set aside to cool in the pot. Season to taste if required. Cool to room temperature, and ideally refrigerate overnight before using as a filling for the pies.