Prepare the dredge: In a plastic or zip-top bag, combine the flour, salt, and pepper. Mix well to blend the seasonings evenly throughout the flour.
Slice and coat the liver: Cut the beef liver into 1/2-inch thick slices. Add the slices to the flour mixture in the bag, seal it, and shake gently to coat each piece thoroughly. This step helps mellow the liver’s flavor and creates a slight crust when cooked.
Sauté the onions: In a large skillet, heat 3 tablespoons of butter and a drizzle of oil over medium-high heat. Add the sliced onions and cook, stirring occasionally, until tender and slightly golden with a glossy sheen—about 8–10 minutes.
Season and set aside: Transfer the sautéed onions to a dish and sprinkle them lightly with additional salt and pepper. Set aside while you cook the liver.
Cook the liver: Add 4 more tablespoons of butter and a little more oil to the same skillet. Once hot, add the floured liver slices in a single layer. Cook for about 5 minutes, flipping once, until browned on both sides and just cooked through. Do not overcook to avoid toughness.
Combine with onions: Return the sautéed onions to the skillet with the liver. Sprinkle in the minced sage and toss gently to heat everything through and meld the flavors, about 2 minutes.
Make the pan sauce: Remove the liver and onions to a serving plate. To the hot skillet, add the beef stock and dry white wine. Scrape up any browned bits from the bottom as you stir. Simmer until the liquid reduces and thickens slightly into a rich sauce, about 3–5 minutes.
Plate and garnish: Pour the reduced sauce over the liver and onions. Sprinkle with fresh chopped parsley for a burst of color and herbal freshness.
Serve warm: Enjoy with creamy mashed potatoes, buttered rice, or rustic bread to soak up the flavorful pan sauce.
