Making this recipe for the third time tonight—can’t get enough of that amazing sauce

1 pound ground beef
1 egg
1/4 cup minced onion
1/4 cup breadcrumbs
2 cloves garlic, crushed
1 teaspoon salt
1/2 teaspoon black pepper
1 teaspoon dried thyme
1 teaspoon dried rosemary
1 tablespoon olive oil
1 large onion, sliced
8 ounces mushrooms, sliced
2 cups beef broth
1 packet brown gravy mix
2 tablespoons ketchup
1 tablespoon Dijon mustard
1 tablespoon Worcestershire sauce
2 tablespoons cornstarch
2 tablespoons water