Pat the beef cubes dry with paper towels and season generously with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Add the beef in a single layer (work in batches if necessary) and brown on all sides. Transfer the seared beef to the bowl of a 6-quart or larger slow cooker.

To the same skillet, add the chopped onion, carrots, and celery. Cook, stirring occasionally, until softened, about 5-7 minutes. Add the minced garlic and cook for one more minute until fragrant.

Pour the red wine into the skillet to deglaze, scraping up any browned bits from the bottom of the pan with a wooden spoon. Bring to a simmer and cook for 1 minute. Pour this mixture into the slow cooker.

Add the diced tomatoes, beef broth, tomato paste, dried oregano, and dried rosemary to the slow cooker. Stir everything together until well combined.

Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the beef is tender and falling apart.

Stir in the halved Kalamata olives and fresh parsley during the last 30 minutes of cooking. To serve, ladle the stew into bowls and top with crumbled feta cheese.