Season and coat the beef. Pat the cubes dry with paper towels, then toss them in a large bowl with the salt, pepper and flour until every piece has a thin, dusty coat.
Sear (optional, 10 minutes). Heat the oil in a large skillet over medium-high heat until it shimmers. Brown the beef in 2 batches, 3-4 minutes per side, until a deep brown crust forms; don't crowd the pan or the meat will steam. Skip this step on a true lazy day: the stew will be lighter in color and flavor, but just as tender.
Load the slow cooker. Spread the potatoes, carrots and onion in the bottom of a 6- to 7-quart slow cooker. Set the beef on top and add the thyme and bay leaves.
Build the liquid. Whisk the garlic, tomato paste, Worcestershire, wine and broth together. If you seared, pour this into the hot skillet instead and simmer 1-2 minutes, scraping up the browned bits. Pour it over the beef; the liquid should come about three-quarters of the way up the ingredients, not cover them.
Cook low and slow. Cover and cook on LOW for 8-9 hours or on HIGH for 5-6 hours. Keep the lid closed for at least the first 6 hours on LOW (4 on HIGH).
Test, don't trust the clock. Press the largest cube with a fork and twist. It's ready when it splits apart with almost no resistance. If it feels springy or squeaks between your teeth, cover and cook 45-60 minutes more, then test again.
Thicken. Discard the bay leaves and skim off the fat pooled on top. Stir in the cornstarch slurry and the frozen peas, switch to HIGH, cover and cook 15-20 minutes, until the gravy turns glossy and coats the back of a spoon.
Rest and serve. Turn the cooker off, taste and add salt as needed, stir in the parsley and let the stew stand 10 minutes so the gravy settles.