Combine the finely minced onions, shredded frozen butter, minced garlic, Worcestershire sauce, mustard, and ground beef. Mix gently to avoid overworking the meat, which can lead to tougher steaks. Form the mixture into oval-shaped patties of equal size, then refrigerate for about 15 minutes to allow them to firm up. Meanwhile, gather and measure out the remaining ingredients.

Remove the chilled patties from the fridge and season both sides generously with salt and pepper. In a large skillet, heat the olive oil over medium-high heat. Sear the steaks for 2-3 minutes on each side, turning them carefully. Set the cooked patties aside on a plate.

Lower the heat to medium-low and melt 2 tablespoons of butter in the same skillet. Add the sliced onions and cook for 15-20 minutes, stirring occasionally, until they soften and begin to caramelize. Pour in the chicken broth and beef broth, then sprinkle in the garlic powder, onion powder, and Worcestershire sauce. Use a spatula to scrape up any browned bits from the bottom of the skillet for added flavor.

In a small bowl, whisk together the cornstarch and cold water until smooth. Gradually whisk this mixture into the simmering gravy. Once the gravy thickens, reduce the heat to medium-low.

Return the seared steaks to the skillet and spoon the gravy over them. Cover and let the steaks cook in the gravy for an additional 10 minutes, ensuring they reach an internal temperature of 160°F.

Before serving, stir in the remaining tablespoon of cold butter to give the gravy a silky, velvety texture. Garnish with parsley and serve the steaks with mashed potatoes and roasted carrots.