Mince half of the onion and slice the other half. Slice the mushrooms. Microwave minced onion for 1 minute at 600W, drain excess water, and let cool.

Combine breadcrumbs and milk in a bowl to moisten.

Mix ground meat, minced onion, salt, and pepper into the breadcrumb mixture until sticky. Divide into two portions and shape into oval patties.

Heat salad oil in a pan over medium heat. Cook patties for 2 minutes on each side until browned. Add sliced onion and mushrooms to the pan and sauté until softened.

Add water and beef stew roux to the pan. Stir until thickened, cover, and simmer on low heat for 7–10 minutes. Occasionally baste the patties with sauce. Remove the lid and reduce liquid. Add butter and let it melt with residual heat.