1 large or 2 small pork tenderloins, about 1 1/2 pounds
2 tablespoons butter, divided
2 tablespoons olive oil, divided
2 medium onions, thinly sliced
1 teaspoon sugar
8 ounces sliced mushrooms
1/3 cup flour
1 teaspoon salt
1/4 teaspoon ground black pepper
1/2 cup red wine
Slice the pork crosswise, about 3/4 to 1-inch in thickness. With the heel of your hand or a meat tenderizer, gently pound to make the medallions about 1/2-inch thick.
In a large skillet, melt 1 tablespoon of butter with 1 tablespoon of olive oil over medium heat. Sauté the onions, stirring frequently, until softened. Add the sugar and flour - continue cooking, stirring frequently, for about 15 minutes. Add the mushrooms to the onion mixture and continue cooking for about 5 minutes, until the mushrooms are tender and golden brown. Remove the onions and mushrooms to a plate and set aside.
Sprinkle the pork with the salt and pepper; dredge in the flour. Add the remaining 1 tablespoon of butter and 1 tablespoon of oil to the skillet. Cook the pork medallions over medium heat for about 3 to 4 minutes on each side, or until nicely browned. Add the wine to the skillet along with salt, pepper and the onion and mushroom mixture. Simmer, uncovered, for 2 minutes. Cover and reduce heat to low; cook for about 10 minutes longer.
Serve the pork tenderloin with hot cooked rice, potatoes, or pasta.