Prepare the Beef: The beef stew meat is patted dry with paper towels. If using flour, the meat is lightly coated in flour.
Brown the Meat: Vegetable oil is heated in a skillet over medium heat. The beef is seared in batches until browned (3–4 minutes per side), then transferred to a crock pot.
Deglaze the Skillet: Beef stock is poured into the skillet, and the browned bits at the bottom are scraped to release flavor. The liquid is then added to the crock pot.
Layer Vegetables: The onion, garlic, celery, leek, fresh carrots, frozen peas and carrots, potatoes, mushrooms (if using), and diced tomatoes are placed in the crock pot.
Season: Basil, rosemary, seasoning salt, and oregano are sprinkled evenly over the mixture.
Cook: The crock pot is set to high heat and cooked for 3 hours.
Thicken (Optional): If a thicker stew is desired, a slurry of cornstarch and water is stirred into the stew during the final 30 minutes of cooking.
Serve: The stew is ladled into bowls and served hot.