In a large bowl, combine the ground beef, breadcrumbs, chopped onion, egg, Worcestershire sauce, Dijon mustard, garlic powder, onion powder, salt, and pepper.

Use your hands to gently mix all the ingredients together until just combined. Be careful not to overmix, as this can make the patties tough.

Divide the mixture into 6-8 equal portions and form them into oval-shaped patties, about 1/2-inch thick.

Heat the olive oil in a large skillet over medium heat.

Add the sliced onion and cook until softened and lightly golden, about 5-7 minutes.

Add the minced garlic and cook for another minute until fragrant.

Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Bring to a simmer, scraping up any browned bits from the bottom of the skillet.

In a small bowl, whisk together the flour (or cornstarch) and cold water until smooth.

Slowly pour the mixture into the simmering gravy, whisking constantly to prevent lumps. Simmer for a few minutes until the gravy has thickened slightly. If you prefer a thinner gravy, you can skip this step entirely.

Pour the gravy into the bottom of your slow cooker.

Gently place the Salisbury steak patties on top of the gravy, making sure they are mostly submerged.

Sprinkle with dried thyme and rosemary.

Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours. The Salisbury steak patties are done when they are cooked through and tender.