1
Pat the chuck cubes bone-dry with paper towels. Moisture kills the sear. Toss with salt, pepper, and flour until every side is coated.
2
Heat oil in a cast iron skillet over high heat until it shimmers. Sear beef in 2 or 3 batches, spaced apart, 2 to 3 minutes per side without moving. You want deep mahogany, not grey. Transfer to the slow cooker.
4
Add onion, garlic, carrots, celery, potatoes around the beef. Stir in the broth, tomato paste, Worcestershire, paprika, thyme, rosemary, bay leaves. Liquid should reach two-thirds up the solids, not fully submerged.