Add potatoes, carrots, and thyme to the slow cooker.

Pat dry the chuck roast with a paper towel. In a small bowl, combine 1 1/2 teaspoons salt and 1 teaspoon pepper; evenly season all sides of the roast.

Heat a large sauté pan over medium-high heat. Add olive oil, once hot, add the beef to the pan. Sear the until golden brown, about 5 minutes per side. Transfer to the slow cooker and reserve the pan and drippings.

Heat pan to medium and add onions, sauté until lightly golden and tender, about 2 minutes. Add garlic and sauté for 30 seconds.

Add balsamic vinegar and sauté until reduced and coats the onions, about 1 minute.

Add tomato paste, stir and cook for 1 minute. Transfer onion mixture to the slow cooker.

Heat pan over medium-high heat, add butter and flour, whisk to combine, cook for 30 seconds.

Gradually add beef stock, vigorously whisking to create a smooth and lightly thickened sauce, about 1 to 2 minutes. The liquid should be near a boil to thicken, increase the heat if needed once all of the stock is added.

Add soy sauce and 1/2 teaspoon salt to the sauce, taste and adjust seasoning as needed. Add sauce to the slow cooker.

Place the lid on the slow cooker and cook on HIGH for 4 to 5 hours or LOW for 7 to 8 hours.