In a large bowl, combine ground beef, egg, minced onion, breadcrumbs, crushed garlic, salt, pepper, thyme, and rosemary. Mix until well combined.
Shape the mixture into 4 patties.
Heat olive oil in a skillet over medium heat. Brown the patties on both sides, about 3-4 minutes per side.
Place sliced onions and mushrooms in the bottom of the slow cooker to create a bed.
Arrange the browned patties on top of the onions and mushrooms.
In a separate bowl, whisk together beef broth, brown gravy mix, ketchup, Dijon mustard, and Worcestershire sauce.
Pour the gravy mixture over the patties and vegetables in the slow cooker.
Cover and cook on low for 5-6 hours, or until the patties are cooked through and tender.
Remove the patties from the slow cooker and set aside.
In a small bowl, mix cornstarch with water to create a slurry. Stir this into the gravy in the slow cooker to thicken.
Return the patties to the slow cooker, coating them in the thickened gravy before serving.
Variations & Tips

For a lighter version, you can use ground turkey instead of beef. If you prefer a spicier kick, add a pinch of cayenne pepper to the beef mixture. For a gluten-free option, use gluten-free breadcrumbs and gravy mix. You can also experiment with different herbs, such as oregano or basil, to suit your taste preferences.