Trim any excess fat from the beef sirloin and cut into 1-inch cubes.
Heat the avocado oil in a heavy-bottomed pot or Dutch oven over medium-high heat.
Season the beef cubes with sea salt and black pepper, then add to the pot to sear until browned on all sides.
Add the diced onion, celery, sliced carrots, and diced parsnips to the pot, sautéing for about 5 minutes until the vegetables begin to soften.
Stir in the tomato paste and dried thyme, cooking for 1 minute to allow the flavors to bloom and deepen.
Pour in the beef bone broth and add the bay leaf, using a wooden spoon to scrape up any flavorful browned bits from the bottom of the pot.
Bring the liquid to a gentle boil, then immediately reduce the heat to low.
Cover the pot and simmer for 45 to 60 minutes, or until the beef is tender and the vegetables are cooked through.
Remove the bay leaf and serve the stew hot in a large bowl.