Make Panade: In a medium bowl, combine bread with milk and soak well, breaking up bread pieces with your fingers until no firm or dry bits remain. Set aside.

Make Steaks: In a large mixing bowl, combine ground beef, ground pork, minced yellow onion, egg yolks, salt, black pepper, onion powder, garlic powder.. Add bread mixture.Mix well until all ingredients are thoroughly combined.

Form into 4 oblong, steak-like shape about 3/4 inch thickt. Using a finger, form dimples all over top side of each ground-meat steak.

In a large cast iron skillet, heat oil over medium-high heat until shimmering. Add ground-meat steaks, dimpled side up, and lower heat to medium. Cook, rotate steaks for even browning, until browned on one side, about 8 minutes. Flip steaks and repeat on other side until browned, about 8 minutes longer. Continue cooking to 145°F. Transfer steaks to a platter.

Make Gravy: In a measuring cup or medium bowl, combine cornstarch with a couple tablespoons chicken stock, stirring to form a smooth slurry with no lumps. Add remaining stock, stir well, and set aside.

Increase heat to medium-high and add mushrooms to skillet, scraping and stirring until mushrooms release their liquid and you can scrape up browned bits on bottom of pan, about 1 minute. Add diced onion and cook, stirring, until onion releases its liquid and you can scrape up browned bits from mushrooms on bottom of pan, about 1 minute. Continue cooking until onion liquid evaporates and onion is softened and beginning to turn lightly golden, about 2 minutes longer.

Stir in ketchup or tomato paste and cook, stirring, for 30 seconds. Stir stock mixture to lift any settled cornstarch from the bottom, then add to skillet and bring to a simmer, stirring and scraping up any browned bits. Continue to cook until stock has reduced by about one-third and has thickened slightly.

Stir in Worcestershire sauce and season with salt and pepper. Lower heat and add butter, stirring vigorously, until butter has melted and a smooth, emulsified sauce forms that lightly coats the back of a spoon. Add just enough vinegar to taste to balance out the sauce. (Use small increments of about 1/2 teaspoon, tasting after each addition to ensure you dont add too much.)

Return steaks to skillet along with any accumulated juices, spoon sauce all over, and simmer gently until warmed through. Serve right away.