Shape mixture into 4 equal patties and then oval shape; place them in a plate. Cover with plastic wrap and transfer to the refrigerator for 1 hour.
In a hot large skillet over medium heat, add oil and when hot, place beef patties; cook 3 minutes per side. Remove from the skillet and place them in a plate, covered loosely with foil.
Return skillet on burner and add red onions; sauté for 2 minutes. If there’s not enough fat, add a little oil. Add mushroom, sprinkle a pinch sea salt and sauté for 4 minutes. Pour in red wine and cook until liquid is almost evaporated. Pour in beef broth and freshly ground black pepper. Bring to a boil and then reduce to medium; simmer for 5 minutes. Place beef patties back in the skillet, reduce heat to medium low, and simmer until heat through, about 5 minutes.
Move beef patties to serving plates, increase heat to medium-high and pour in cornstarch mixture; stir and less than a minute, the sauce will thicken. Spoon gravy over patties and sprinkle with fresh chopped parsley.