Heat a large skillet over medium-high heat and add olive oil. Sear the beef cubes in batches until browned on all sides, about 5 minutes per batch—this locks in flavor and creates a rich base.
Transfer the browned beef to your slow cooker. Toss in the chopped carrots, sliced parsnips, diced turnip, and diced onion for a colorful, hearty mix.
In a small bowl, whisk together beef broth, tomato paste, minced garlic, thyme, and a pinch of salt and pepper. Pour this mixture over the ingredients in the slow cooker, ensuring all are submerged.
Cover the slow cooker with the lid and cook on low for 8 hours, allowing the flavors to meld and the beef to become melt-in-your-mouth tender. The vegetables will soften and absorb the savory juices.
Once cooked, remove the lid and gently stir the stew. If you prefer a thicker broth, remove the meat and vegetables, then turn the slow cooker to high. Mix a teaspoon of cornstarch with a little cold water and stir into the hot liquid until it thickens.
Return the meat and vegetables to the thickened broth, stir gently to combine, and cook for another 15 minutes until heated through and slightly more concentrated.
Taste and adjust seasoning with additional salt, pepper, or herbs if desired. The stew should be rich, hearty, and simmering with a velvety, thickened sauce.
Serve hot in bowls, garnished with fresh herbs if you like. The stew will have a rustic appearance with tender meat, glossy vegetables, and a thick, flavorful broth—perfect for cozy days.