Toss the beef cubes with the flour, salt, and pepper until evenly coated.
Heat a large skillet over medium-high heat and brown the beef in batches, then transfer it to the slow cooker.
Add the sliced leeks, halved mushrooms, sliced carrots, and minced garlic to the slow cooker.
Stir in the red wine, beef broth, tomato paste, dried thyme, and bay leaves until the tomato paste dissolves.
Cover and cook on low for 8 hours until the beef is fork-tender and the gravy has thickened.
Remove the bay leaves, taste and adjust salt and pepper if needed, then sprinkle with chopped parsley before serving.