Prep the beef: Pat the beef dry with paper towels. Toss with salt, pepper, and flour until lightly coated.
This helps create a crust and thickens the stew as it cooks.
Brown in batches: Heat oil in a large skillet over medium-high. Sear the beef in two or three batches, 2–3 minutes per side, until browned. Transfer to the slow cooker.
Don’t overcrowd the pan.
Sauté the aromatics: In the same skillet, add onion and a pinch of salt. Cook 3–4 minutes until softened, scraping up browned bits. Stir in garlic and tomato paste; cook 1 minute until fragrant.
Deglaze with Guinness: Pour in half the Guinness, scraping the pan to release any stuck-on bits.
Let it bubble for 1–2 minutes, then pour the mixture into the slow cooker.
Load the veggies: Add potatoes, carrots, and celery to the slow cooker. Pour in the remaining Guinness and the beef broth. Stir in Worcestershire, thyme, and bay leaves.
Cook low and slow: Cover and cook on Low for 8–9 hours or High for 4–5 hours, until the beef is fork-tender and the vegetables are soft but not mushy.
Finish and adjust: Remove bay leaves.
If you want a thicker stew, stir together cornstarch and water, then mix it into the stew. Cook on High for 15 minutes until slightly thickened.
Add peas and garnish: Stir in peas during the last 10 minutes for a pop of color and sweetness. Taste and adjust salt and pepper.
Sprinkle with parsley before serving.
Serve: Ladle into warm bowls. It’s excellent with crusty bread or over creamy mashed potatoes.