Prep the beef: Pat the beef dry with paper towels. Season generously with salt and pepper.
Toss with the flour to lightly coat. This helps with browning and thickening later.
Brown the beef: Heat oil in a large skillet over medium-high heat. Sear the beef in batches until browned on two sides, 2 to 3 minutes per side.
Don’t crowd the pan. Transfer browned beef to the slow cooker.
Sauté aromatics: In the same skillet, add the onion with a pinch of salt. Cook 3 to 4 minutes, scraping up browned bits.
Stir in garlic for 30 seconds. Add tomato paste and cook 1 minute to caramelize. Transfer everything to the slow cooker.
Add vegetables: Add carrots, celery, potatoes, and mushrooms (if using) to the slow cooker over the beef.
Build the broth: Pour in beef broth and Worcestershire.
Add thyme, rosemary, smoked paprika (if using), and bay leaves. Stir gently to combine.
Cook low and slow: Cover and cook on Low for 8 to 10 hours or on High for 4 to 5 hours, until the beef is very tender and the vegetables are cooked through.
Thicken the stew: Stir together cornstarch with 2 tablespoons cold water. Stir the slurry into the stew.
Add peas. Cook on High for 15 minutes, uncovered or partially covered, until the gravy thickens slightly.
Finish and serve: Remove bay leaves. Taste and adjust seasoning with salt and pepper.
Sprinkle with chopped parsley. Serve with crusty bread or over mashed potatoes.