Although it's made with just a handful of ingredients, this pork tenderloin looks (and tastes!) impressive. Pork tenderloin is a very lean cut, meaning this simple recipe is healthy too. You can use any combination of onions you like, as long as you have about 8 cups in total.
Cut the pork tenderloins in half crosswise. In a large bowl, toss the pork with 2 tablespoons of the olive oil and 1 teaspoon salt.2.
Heat a large skillet over medium-high heat. When the skillet is hot, add the pork (without adding any oil to the skillet) and sear on all sides, about 8 minutes. Reduce the heat to medium, and continue to cook the pork, turning occasionally, until the internal temperature reads 145º F on an instant-read thermometer, about 10 to 15 minutes. Let the pork rest 5 minutes on a cutting board before slicing.3.
Meanwhile, in another large skillet over medium-high heat, heat the remaining 3 tablespoons olive oil. When the oil is hot, add the yellow onions, red onions, and shallots. Season with the remaining 1/2 teaspoon salt, and toss in the bay leaves. Cook and stir until the onions are completely wilted and golden, about 15 minutes.4.
Reduce the heat to medium, and add the balsamic vinegar and 1/4 cup water. Cover, and simmer until the onions are tender, about 10 minutes. Uncover, increase the heat to medium high, and cook, stirring, until the liquid is reduced to a syrupy glaze.5.
Remove the bay leaves and keep the onions warm while you slice the pork. Slice the pork into 1-inch-thick slices against the grain. Serve the pork on a bed of the balsamic onions.