Cook the bacon in a large skillet until crisp. Transfer the bacon to a dish and set aside.
Season the chuck roast with salt and pepper and sear it in the bacon fat over medium-high heat until brown on all sides, 2-3 minutes per side. Transfer the browned roast to the insert of your slow cooker.
Saute the onions (and optional celery) in the remaining bacon fat over medium heat for 3-4 minutes or until they start to soften.
Deglaze the pan with the beer, scraping up any bits stuck to the bottom. Transfer the liquid and onions to the slow cooker.
Add the mustard, thyme, bay leaves, chopped dill pickle, and chopped bacon to the slow cooker, give it a stir to mix, and set it on high for about 2 hours.
Add the baby potatoes, carrots, pickles, reserved chopped bacon, and red cabbage to the slow cooker after 2 hours, working the veggies into the liquid as much as possible.
Cook an additional 2-3 hours or until the beef is fall-apart tender.
Transfer the meat to a cutting board and tent to keep warm. Transfer the vegetables to a bowl to keep warm.
Make a gravy by mixing 2 tablespoons of all-purpose flour with 1/4 cup of water or beef broth and whisk this into the liquid in the slow cooker. Turn on high to simmer and thicken.
Temper the sour cream before adding it to the gravy. To do this, add 1/4 cup of gravy to the sour cream and stir it in to raise the temperature of the sour cream, then whisk the sour cream into the gravy. Do not boil the gravy with the sour cream in it or it might break and curdle.
Season the gravy with salt and pepper. Slice the beef and serve with the veggies and gravy.