Place onions, garlic, breadcrumbs, yogurt, ketchup, beef bouillon, Worcestershire, mustard, Italian seasoning, salt, and pepper in a large mixing bowl, mix until well combined. Add ground beef; mix until well combined.

Shape into 6 oval patties, around 3/4" inch thick (2 cm). Notes: This is optional, but I like to sprinkle 1 tablespoon of all-purpose flour over the patties; this helps them to form a nice crust and also to thickens the sauce.

In a large skillet, add the olive oil and butter and heat until melted over medium-high heat. Add the meat patties and fry on both sides until no longer pink in the middle, about 4 - 5 minutes per side. Transfer the patties to a plate. Pour off any excess grease if necessary.

Reduce the heat to medium. In the same skillet, add 2 tablespoons of olive oil. Add chopped onion and garlic and cook for 2 minutes, or until onions are fragrant and translucent.

Add the mushrooms into the skillet and cook for 2 - 3 minutes until golden.

Add the beef broth, mustard, soy sauce, and Worcestershire sauce and stir to combine.

Then, in a small bowl, whisk the cornstarch with a little beef broth or water and add to the skillet; whisk until the sauce thickens, for about 2 minutes.

Add the meat patties back to the skillet and spoon gravy over the top. Cook for another minute until the meat patties warm through.

Garnish with parsley and serve over mashed potatoes.