Heat olive oil in a large skillet over medium-high heat until shimmering. Working in batches, sear the cubed beef for 2-3 minutes per side until deeply browned, developing the caramelization that gives this stew its rich foundation.

Transfer browned beef to your slow cooker. In the same skillet, sauté diced onion for 4-5 minutes until softened and golden, then add minced garlic and cook 30 seconds until fragrant.

Add tomato paste to the onion mixture and stir constantly for exactly 2 minutes, allowing it to caramelize slightly before adding 1 cup red wine to deglaze the pan, scraping all browned bits.

Pour the wine mixture into the beautiful slow cooker beef stew recipe base along with beef broth, sliced carrots, potatoes, mushrooms, thyme, rosemary, and bay leaves. Season with salt and pepper, then stir well.

Cover and cook on low for 8 hours, checking at the 7-hour mark; the beef should shred easily with a fork when ready. Never cook beyond 8.5 hours or meat becomes stringy.

Mix cornstarch and water into a slurry, then stir into the slow cooker for the final 15 minutes on high to thicken the sauce to silky consistency. Taste and adjust seasonings before serving.