Heat the oil in a large stock pot over medium high heat. Toss the beef with a sprinkle of salt and pepper, then toss them with the flour so all the pieces are coated.
Add the beef to the pot and sear until each side is golden brown, about 1 to 2 minutes per side. You may need to do this in batches. Remove the beef with a slotted spoon and place them on a plate until ready to use.
Add the butter to that same pot. Add the onions with a big pinch of salt. Stir. Cook, stirring often, for 10 minutes until the onions softened. Reduce the heat to low. Cook for 1 hour, stirring often, until the onions are caramely.
Once caramelized, stir in the carrots, celery and mushrooms with a pinch of salt. Cook for 5 to 6 minutes.
Add in the potatoes and the beef. Add in the stock and thyme. Bring the mixture to a boil then reduce it to a simmer. Cover and let it simmer for 1 to 1.5 hours. The meat should be tender once done.
If you’d like it to be thicker at this point, you can always make a slurry! I take 1 cup of beef stock and 1 tablespoon of flour and shake them together for 30 seconds. Then I pour it into the pot and bring it to a boil until thickened, about 5 to 6 minutes.
Time to serve! Remove the sprigs of thyme and taste, seasoning more if needed. Grab bowls and serve immediately.