In a large bowl, gently combine ground beef, 1/2 cup breadcrumbs, egg, finely chopped onion, 2 cloves minced garlic, 1 tsp Worcestershire, Dijon (if using), salt, and pepper. Don’t overwork the meat — mix until just combined.

Shape the mixture into 4 even patties (about 4–5 oz / 110–140 g each). Press a slight indentation into the center of each patty.

Heat 1–2 tablespoons olive oil or butter in a large skillet over medium-high heat. Once shimmering, add patties and brown 4–5 minutes per side until a deep golden crust forms.

Remove patties to a plate and tent with foil.

Reduce heat to medium. In the same skillet, add 1 tablespoon butter and the sliced mushrooms. Cook 5–6 minutes until mushrooms release liquid and start to brown.

Add the chopped shallot/onion and cook 2–3 more minutes until softened.

Sprinkle 1 tablespoon flour over the mushrooms and stir to coat (or stir in cornstarch slurry now). Cook 1 minute to remove raw flour taste.

Gradually pour in 2 cups beef broth while stirring or scraping up browned bits from the pan. Add 1 tbsp Worcestershire and 1 tbsp soy sauce/tamari (if using). Bring to a gentle simmer.

Return the browned patties to the skillet, spoon gravy and mushrooms over them, cover, and simmer on low for 8–12 minutes until patties reach 160°F (71°C) and the sauce has thickened.

Taste and adjust seasoning with salt and pepper. Sprinkle chopped parsley over the top and serve hot.