Rinse and cook the Japanese Short-Grain Rice according to package instructions, then keep warm.
Slice the Onion thinly, clean and slice the Mushroom, and cut the Beef into bite-sized strips; pat the beef dry and season lightly with Salt and freshly ground Black Pepper.
Heat a wide skillet over medium-high heat and add a splash of Neutral Oil. Sear the beef in batches until browned but not fully cooked through, then transfer to a plate and reserve any juices.
Reduce the heat to medium, add a knob of Unsalted Butter to the pan, then add the Onion and cook until soft and caramelized. Add the Mushroom and sauté until they release their liquid and begin to brown.
Sprinkle the All-Purpose Flour over the vegetables, stir to coat and cook for a minute to eliminate the raw flour taste.
Carefully pour in the Red Wine to deglaze the pan, scraping up the browned bits from the bottom.
Stir in the Beef Stock, Tomato Paste, Ketchup, Worcestershire Sauce, and add the Bay Leaf and a pinch of Sugar. Bring to a simmer and reduce gently until the sauce thickens and flavors concentrate.
Return the seared Beef and any accumulated juices to the pan, simmer briefly until the beef is cooked through and the sauce coats the meat. Taste and adjust seasoning with additional Salt and Black Pepper as needed.
Finish the sauce with another small piece of Unsalted Butter to add shine and roundness, remove the Bay Leaf, and spoon the beef and sauce over the warm Japanese Short-Grain Rice. Serve immediately.
