In a large bowl, add the beef, the breadcrumbs, the tomato paste, 1 teaspoon of the Worcestershire sauce, the ground mustard, the onion powder, 1/2 teaspoon of the pepper, and the salt and using your clean hands, mix gently but thoroughly until well-combined.
Divide the beef mixture into 4 equal-sized portions and form them into oval patties about 1/2-inch-thick.
In a large, cast-iron or heavy-bottomed skillet over medium heat, add the olive oil.
Add the patties to the pan and cook until browned on the bottom, about 2-3 minutes.
Flip the patties and cook until browned on the reverse side but not cooked through, about 3-4 minutes.
Transfer the patties to a plate or platter and tent them with foil to keep them warm.
Drain the excess fat from the skillet and return the skillet to the stovetop over medium heat.
Add the onions to the hot skillet and cook until translucent, about 2-4 minutes.
Add the mushrooms and cook until softened, about 3-5 minutes.
Add 1 teaspoon of the thyme, the garlic powder, and the flour and cook until the raw flour smell disappears, about 1 minute.
Slowly add the beef stock to the onion mixture, while scraping the bottom of the skillet to loosen any browned bits.
Add the remaining Worcestershire sauce to the onion mixture and stir to combine.
Bring the onion mixture to a boil and let it boil until it begins to thicken into a gravy.
Reduce the heat to a simmer.
Add the butter to the gravy and stir until melted and combined.
Season the gravy with the salt and the extra black pepper.
Add the beef patties to the gravy and simmer until they are no longer pink in the middle and reach an internal temperature of 155 degrees F for well done, about 5-7 minutes.
Plate the patties over the mashed potatoes and drizzle with the gravy.
Garnish with the extra thyme and serve.