Cut or snip bacon slices into small pieces and cook till crisp in a large pot. Remove bacon to a paper towel lined plate and set aside.
Season beef chunks well with salt, pepper, onion powder, Coating all sides.
Sauté beef in the bacon fat until browned, remove to bowl or plate. You may need to do this in two batches depending on your pot size. Remove beef and most of liquid to a bowl.
Once beef is removed, add onions, chopped carrots, celery, leeks and thyme to the pot and sauté until just browned. Scraping up any browned bits (called fond) from the pot and stirring often. Add garlic and sauté another minute or so. Feel free to add a nob of butter to mixture while sautéing if you need more fat.
Add in the tomato paste.
Add in flour and and cook for 2 minutes stirring often.
Add beef cubes back in as well any liquid removed or accumulated on the plate/bowl. Get all that tasty goodness in the pot.
Pour in the Guinness, beef stock and Worcestershire. Stir well.
Add in your thyme branches.
Add in your carrot slices here (as well as potato chunks if using, rutabaga, parsnips or any other root vegetables. If you enjoy onion, slice up another onion and slide rings in here as well.)
Bring mixture to a simmer, reduce heat to low and cook for 2 hours. Checking every so often. (If you add mushrooms, add then in with ½ hour to go).
Remove the Thyme bunch/branches. Add in scallions if you like here as well. Taste and if necessary add salt. Definitely add in black pepper to taste.
If you want your stew thicker than it is, scoop out ½ cup of liquid and stir in 1 -2 TBS flor, slowly add it back in stirring until it comes to a simmer again.