Prepare the liver: Rinse the beef liver under cold water and pat it thoroughly dry with paper towels. Slice the liver into 1/2-inch thick pieces. If there are any tough membranes or veins, carefully trim them away with a sharp knife.

Dredge the liver: In a shallow dish, combine the flour, salt, pepper, garlic powder, and onion powder. Lightly coat each piece of liver in the flour mixture, shaking off any excess.

Sear the onions: Heat 1 tablespoon of butter and 1 tablespoon of olive oil in a large, heavy-bottomed skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they are softened and caramelized, about 8-10 minutes. Add the minced garlic during the last minute of cooking and stir until fragrant. Remove the onions and garlic from the skillet and set aside.

Cook the liver: Add the remaining 2 tablespoons of butter and 1 tablespoon of olive oil to the same skillet over medium-high heat. Once the butter has melted and the oil is shimmering, carefully place the floured liver pieces in the skillet in a single layer, ensuring not to overcrowd the pan (work in batches if necessary).

Sear for 2-3 minutes per side, until golden brown and cooked through. The liver should still be slightly pink in the center for optimal tenderness. Avoid overcooking, as this will make the liver tough.

Combine and serve: Return the cooked onions and garlic to the skillet with the liver. Toss gently to combine. Serve immediately.