Dry the chuck with paper towels, season every surface with salt and pepper, and heat a large skillet over medium-high heat. A dry surface encourages a browned crust instead of a wet, gray exterior.

Melt the butter with the avocado oil in the skillet, then brown the roast for about 2 to 3 minutes on each side before transferring it to a plate. Listen for a steady sizzle and look for a deep brown crust with a toasted beef aroma.

If desired, add a little more oil to the skillet and sauté the sliced onions for 3 to 4 minutes. The onions should begin to soften and turn translucent while picking up some of the browned pan flavor.

Arrange half the onions in the cooker, add the garlic, beef stock, Worcestershire sauce, apple cider vinegar, onion powder, dried thyme, bay leaf, celery seeds, and sugar, then place the roast on top and cover it with the remaining onions. The two onion layers surround the beef with flavor as the onions release moisture and soften.

Cover the cooker and cook on Low for 6 to 8 hours, checking the roast for tenderness toward the end. A fork should enter the center easily and separate the fibers with a gentle twist.

Transfer the tender beef to a platter and cover it to keep it warm while you whisk 2 tablespoons cornstarch with the ⅓ cup cold water, stir the mixture into the cooking liquid, and cook for another 5 to 7 minutes. The gravy should become smoother and thicker as the starch heats through.

If the gravy remains thin, dissolve the remaining tablespoon of cornstarch in 2 tablespoons of water, stir it into the cooker, and cook for an additional 5 minutes. Stop when the gravy lightly coats a spoon rather than running off like stock.

Keep the beef covered while the gravy finishes, allowing it to rest before you pull it apart. Serve with cheddar mashed potatoes, or offer crusty bread and a house salad alongside.