In a 5-quart pot with a lid, heat the oil on medium-high heat. When the oil starts to ripple, add half the beef and brown on both sides, about 2 to 3 minutes per side. Remove the browned beef and cook the remaining meat the same way. Add the browned beef back into the pot along with the garlic and rosemary; cook 1 minute, stirring constantly. Add 3 tablespoons all-purpose flour and cook 2 minutes, continuing to stir constantly.
Add 6 cups beef stock, the Worcestershire, bay leaves, salt, and pepper. Turn heat up to high and bring the stew up to a boil, then cover the pot, turn heat down to simmer, and let it cook for 60 to 90 minutes (stirring occasionally), or until the meat is tender but not falling apart.
Add the onion, carrot, celery, and potato and enough water to just cover everything (about 1 to 2 cups). Turn the heat up to high and bring to a boil, then turn heat down to simmer, cover the pot, and cook until the veggies are tender, about 30 minutes, stirring occasionally. If you want your stew even thicker, whisk in the flour/water mixture during the last 10 minutes that the veggies are cooking, stirring more frequently after you add this.
Add the mushrooms and peas and cook 5 minutes more.
Taste and season with additional salt and pepper as desired. Serve hot, on top of mashed potatoes or mashed cauliflower, if desired.