Dry the meat with paper towels and add to a mixing bowl. Season the flour with the salt and pepper, then dust over the meat cubes. Mix well to coat. Set a heavy-bottomed pot over medium-high heat and add 2 tbsp. of the vegetable oil. When the oil is hot, add ¼ of the meat cubes (dust off any excess flour). Brown the meat cubes deeply on at least two sides, then remove to the crock of your slow-cooker and add the next small batch of meat cubes. Repeat until all meat cubes have been browned, adding more oil as necessary between batches to keep the pot from drying out.
Reduce heat to medium and add the diced onion and celery, adding more oil to the pot if needed. Sauté until softened, 4-5 minutes. Add the garlic, rosemary stem and needles, thyme leaves, and bay leaves; sauté an additional minute. Add in the fish sauce. Stir for 30 seconds, scraping any browned bits off the bottom of the pot. Add the Guinness (IF your Guinness contains a nitrogen capsule [you can hear it rattling in the can], allow it to discharge for 10 seconds after cracking the can open). Continue to stir and scrape for 1 minute.
Scoop the contents of the pot into the crock of your slow-cooker along with the beef cubes and any accumulated juices. If there are still browned bits on the bottom of the pot, add the beef stock and continue to scrape. Transfer to the slow-cooker. (Otherwise, add the beef stock directly to the slow-cooker.)
Lid the slow-cooker and set on low for at least 6 hours and up to 8. Meanwhile, steam or par-boil the remaining vegetables until you can pierce them easily with a paring knife (there shouldn't be any resistance at all). Add the vegetables to the stew for the last 30 minutes of cook time (turn the slow-cooker to high; see Recipe Note #3).
Cool the stew by placing the crock of the slow cooker into a sink filled 1/4 of the way with cool water. Remove the stew to an airtight container and refrigerate.