Cook the bacon in a skillet over medium heat until crisp. Remove the bacon and set it aside. Add the beef chunks to the same skillet and brown them on all sides.
Transfer the beef and bacon to the slow cooker. Add the carrots, onion, mushrooms, and garlic.
Stir in the red wine, beef broth, tomato paste, thyme, bay leaves, salt, and pepper. Cover and cook on low for 8 hours.
About 30 minutes before serving, place the potatoes in a large pot and cover with water. Bring to a boil and cook until tender, about 15 minutes.
Drain the potatoes. Add the milk, butter, and a pinch of salt. Mash until smooth and creamy.
Remove the bay leaves from the slow cooker. Spoon the mashed potatoes onto plates and top with the beef bourguignon.