Cook the chopped bacon in a large skillet over medium heat until crisp. Remove the bacon and set it aside, leaving the drippings in the pan.
Season the beef cubes with salt and pepper, then brown them in the same skillet in batches. Transfer the browned beef to the slow cooker.
Add the carrots, onion, and garlic to the skillet and cook for 3 minutes. Stir in the flour and tomato paste, then pour in the red wine and beef broth. Bring the mixture to a simmer.
Transfer the skillet contents to the slow cooker along with the cooked bacon, mushrooms, thyme, and bay leaves. Stir to combine.
Cover and cook on low for 8 hours until the beef is very tender.
About 30 minutes before serving, place the potatoes in a large pot of salted water. Boil until fork-tender, then drain.
Mash the potatoes with the butter and milk until smooth. Season with salt and pepper to taste.
Remove the thyme sprigs and bay leaves from the slow cooker. Serve the beef bourguignon over the mashed potatoes.