Zwiebelrostbraten is sliced medium rare Roast Beef, served with fried onions on top, to which the name refers. It is a traditional dish of Swabian and Austrian cuisine and a classic dish in Vienna restaurants.
In the Swabian region, the Onion roast beef is often served with Spätzle and is considered a Sunday meal. Regionally, it is also sold under the names “Wiener Zwiebelrostbraten”, “Allgäuer Zwiebelrostbraten” or “Schwäbischer Zwiebelrostbraten”.
Preferred is a Top Round Roast, cut similar to the top sirloin in fat and flavor, commonly used for roast beef. I suggest to freeze the meat for about two hours for easier slicing.
For preparation in modern guesthouses, slices of a Top Round beef roast, seasoned with salt and pepper, cut on the edges, turned in flour and possibly rubbed with mustard and garlic, are fried briefly in clarified butter or oil and kept warm. In the same pan, sliced onions, possibly sprinkled with paprika, are roasted and removed. Then add a little flour to the fat in the pan, stir, and extinguish with the stock, broth or wine, so that a slightly creamy sauce is created.
Depending on the recipe, the onions also stay in the sauce, which makes them softer. Finally, meat, sauce and onions are served together with side dishes such as fried potatoes, wide ribbon noodles or like in the Swabian region, Spätzle.