Pat the beef chuck cubes dry with paper towels before seasoning them generously with kosher salt and black pepper. Heat olive oil in a large skillet over medium-high heat, then add half of the beef cubes in a single layer. Sear them on all sides until they're beautifully browned (about 2-3 minutes per side). Once done, set them aside on a plate and repeat with the remaining beef.

Reduce the heat to medium and add chopped onion, carrots, and celery to the same skillet. If needed, drizzle in a touch more olive oil. Sauté these veggies for about 8-10 minutes until they soften and the onion turns translucent. Stir in minced garlic and optional red pepper flakes, cooking for another minute or two until fragrant.

Add tomato paste into the skillet. Cook it for about 2-3 minutes while stirring constantly to deepen the flavor of the tomatoes.

Pour in your apple vinegar and use a wooden spoon to scrape up any browned bits from the bottom of the skillet. Let it simmer for 3-5 minutes until reduced by half.

Transfer the vegetable mixture into the insert of your slow cooker. Layer the seared beef cubes on top, followed by pouring crushed tomatoes, diced tomatoes (with their juice), and beef broth over everything. Toss in bay leaves, dried oregano, dried thyme, and rosemary. Give everything a gentle stir to combine.