Start by patting the beef dry. Season all sides with kosher salt, pepper, and cajun seasoning.
If your slow cooker has a sauté function you can do this all in the slow cooker. If not, we'll start browning the meat in a large pot on the stove first then transfer to a slow cooker.
Over medium-high heat, add 2 tbsp avocado oil and brown the meat, about 4 minutes. Flip and repeat. While the underside browns, add the onion and garlic around the meat to saute for about 1 minute before transferring everything to a slow cooker (including the brown bits).
Sprinkle over the au jus mix, French onion mix, and ranch seasoning. Add in the peppers and their juices then pour over the beef stock. Finish by placing the pats of butter on top the meat.
Cover and slow cook on high for the first hour, then reduce the heat to low and continue cooking until the roast is fork-tender, about 8 hours. Add the carrots halfway through cooking*. It's not finished cooking until it easily shred with a fork with no resistance. Serve over mashed potatoes or buttered noodles.
Some fattier cuts of beef with leave you with a higher fat content in in the pan juices, use a few folded paper towels and tap the surface of the gravy to remove the fat that rises to the top. You can repeat this a few times to remove most the fat.