Prepare and Season the Beef: Pat the 3-pound (1.36 kg) chuck roast dry with paper towels. Trim away any large pieces of hard exterior fat while leaving the internal marbling intact. Season all sides with 1/2 teaspoon (3 g) of the divided kosher salt and 1/2 teaspoon (1 g) of the divided black pepper. The surface should feel relatively dry, which helps the beef develop a browned crust during searing.

Sear the Chuck Roast: Heat 1 tablespoon (15 ml) olive oil in a large skillet over medium-high heat. Add the seasoned chuck roast and sear for 3–4 minutes per side, until the surface develops a deep brown crust. Brown the edges where practical. The beef does not need to cook through at this stage. The goal is to develop flavor on the outside. Transfer the roast to the 6-quart (5.7 L) slow cooker.

Add the Aromatics and Cooking Liquid: Scatter the sliced yellow onion and minced garlic around the roast. Pour in 2 cups (480 ml) low-sodium beef broth and 2 tablespoons (30 ml) Worcestershire sauce. Add the remaining 1/2 teaspoon (3 g) kosher salt and 1/2 teaspoon (1 g) black pepper, followed by 1 teaspoon (1 g) dried thyme and 1 teaspoon (2 g) onion powder. Gently move the liquid around the roast to distribute the seasonings. The beef does not need to be completely submerged.

Slow Cook Until Fork-Tender: Cover the slow cooker and cook on LOW for approximately 7 hours 30 minutes, until the beef is tender enough to pull apart easily. The roast should offer very little resistance when pierced with a fork. If the meat still feels firm or rubbery, it needs additional cooking rather than more liquid. For food safety, whole beef cuts should reach at least 145°F (63°C) followed by a 3-minute rest. However, chuck roast generally needs to cook considerably beyond this minimum temperature to become tender enough for shredding.

Shred the Beef: Transfer the cooked roast to a cutting board or large dish. Using two forks, pull the meat apart into bite-sized shreds, removing any large pieces of fat or tough connective tissue. The beef should separate into moist strands without requiring much force. Return the shredded beef to the slow cooker and stir it into the cooking juices. Cover and keep on the WARM setting while preparing the remaining components.

Cook the Potatoes: Place 2 pounds (900 g) peeled and evenly cut Yukon Gold potatoes into a 4-quart (3.8 L) pot. Add 8 cups (1.9 L) water and bring to a boil over high heat. Reduce to a steady simmer and cook for approximately 15–20 minutes, until the potatoes are tender. A fork should slide through the center of the largest potato piece without resistance. Drain thoroughly and return the potatoes to the warm pot. Let excess surface moisture evaporate briefly.

Mash the Potatoes: Add 4 tablespoons (57 g) cubed unsalted butter to the hot potatoes. Mash until the butter melts and the larger potato pieces break down. Gradually incorporate 1/2 cup (120 ml) warmed whole milk, followed by 1/2 teaspoon (3 g) kosher salt and 1/4 teaspoon (0.5 g) black pepper. Continue mashing just until smooth and creamy. The potatoes should be soft enough to scoop but firm enough to hold their shape on the bread. Avoid vigorous mixing, which can make mashed potatoes gluey. Keep covered and warm.

Prepare the Gravy Base: Carefully ladle the cooking liquid from the slow cooker into a medium saucepan, leaving the shredded beef behind. Include the softened onions and garlic for a rustic gravy, or strain them out if you prefer a smoother finish. Skim off excess surface fat if necessary. Bring the liquid to a gentle simmer over medium heat. In a small bowl, whisk together 3 tablespoons (24 g) cornstarch and 3 tablespoons (45 ml) cold water until completely smooth. The mixture should look milky, without dry lumps.

Thicken the Brown Gravy: While whisking the simmering cooking liquid, slowly pour in the cornstarch slurry. Continue cooking for approximately 2–4 minutes, stirring frequently, until the gravy becomes glossy and thick enough to coat the back of a spoon. If the gravy appears too thin, allow it to simmer briefly while stirring. If it becomes too thick, incorporate a little of the reserved cooking liquid, if available. Return enough gravy to the slow cooker to keep the shredded beef moist, reserving the remainder for serving.

Prepare the Bread: Lightly toast the 6 slices (about 210 g total) of thick-cut white sandwich bread until the surfaces are just firm and faintly golden. Avoid making the bread overly crisp. It should remain tender enough to cut easily with a fork after the gravy is added. Arrange the slices on individual serving plates.

Assemble the Beef Manhattan: Spoon the warm shredded beef over each slice of bread. Add a generous portion of creamy mashed potatoes alongside or partially over the beef, depending on your preferred presentation. Ladle the hot brown gravy over the beef and potatoes, allowing some to soak into the bread. The finished sandwich should be generously sauced without being completely submerged.

Garnish and Serve: If desired, sprinkle the sandwiches with 2 tablespoons (8 g) chopped fresh parsley, divided among the servings. Serve immediately while the beef, potatoes, and gravy are hot. For a classic diner-style presentation, leave the sandwich open-faced and serve with a fork and knife.