Place cut up beef in a large bowl and season with salt and pepper.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat.
Add beef and sear until browned, 2 to 3 minutes per side. Depending on the size of your skillet, you might have to do this in batches, adding more olive oil if necessary.
Transfer the browned beef to a 6-quart slow cooker; arrange prepared carrots, celery, and mushrooms over the meat inside the slow cooker and set aside.
Heat the rest of the olive oil and sauté the onions for 3 minutes, or until softened.
Stir in the minced garlic and continue to sauté for 30 seconds, or until fragrant; transfer to the slow cooker.
Put the skillet back on the stove over medium-high heat and deglaze the pan with red wine. Scrape the bottom of the pan with a wooden spoon to get all the browned bits; cook for 1 minute.
Whisk in the tomato paste and Worcestershire sauce; cook for 30 seconds while whisking.
Slowly whisk in 2 cups of beef broth; cook for 1 minute.
Pour the prepared liquid over the beef and vegetables.
Stir in the remaining broth into the slow cooker and add bay leaves and thyme over the veggies.
Close with a lid and cook on LOW for 7 to 8 hours, or on HIGH for 4 to 5 hours. Times may vary depending on how hot your slow cooker runs.
30 minutes before the beef stew is done cooking, make a slurry by whisking together 2 tablespoons of cornstarch with 3 tablespoons of water. Stir the slurry into the slow cooker, set to HIGH, and continue to cook for 30 minutes.
When done, taste for seasonings and adjust accordingly.
Garnish with parsley and serve.