In a mixing bowl of ample size, blend the ground turkey with the chopped onion, breadcrumbs, egg, Worcestershire sauce, salt, and black pepper until a uniform consistency is achieved.

Shape the turkey mixture into four oval patties, each approximately half an inch in thickness.

In a skillet heated to medium, brown the patties on each side for about 3 minutes. Complete cooking is not necessary at this stage.

Transfer the seared patties to the bottom of a slow cooker.

In another bowl, whisk together the cream of mushroom soup, beef broth, Dijon mustard, ketchup, garlic powder, and dried thyme until smooth and cohesive.

Pour this mixture evenly over the patties in the slow cooker.

Secure the lid and cook on the low setting for 4 to 5 hours, until the patties are fully cooked through and tender to the fork.

Plate the turkey Salisbury steaks with a generous spoonful of the rich gravy and accompany with your favorite side dishes.