In a large bowl, combine the ground beef, breadcrumbs, egg, milk, Worcestershire sauce, onion powder, garlic powder, salt, and pepper.
Divide the mixture into six equal portions and shape into oval patties approximately ¾ inch thick.
Heat olive oil in a large skillet over medium-high heat. Brown the patties for approximately two minutes per side. Remove and set aside.
Add the sliced onion and bell peppers to the skillet. Cook for three to four minutes, then add the garlic and cook for another 30 seconds.
Place half of the vegetable mixture into a 6-quart slow cooker. Arrange the browned patties on top and cover with the remaining vegetables.
Whisk together the beef broth, Worcestershire sauce, tomato paste, Dijon mustard, thyme, and smoked paprika. Use a little of this mixture to scrape the browned bits from the skillet, then pour all the liquid into the slow cooker.
Cover and cook on LOW for three to four hours or HIGH for 1½ to 2½ hours, until the patties reach an internal temperature of 160°F.
Remove the cooked patties. Mix cornstarch with cold water and stir into the gravy. Cover and cook on HIGH for 15 to 25 minutes, until thickened. If needed, thicken the gravy in a saucepan instead.
Return the patties to the gravy. Spoon the peppers and onions over the top, garnish with parsley if desired, and serve warm.