Begin by reducing the reserved carnitas juices. Simmer in a medium saucepan for about 20 minutes, or until reduced by half. Transfer juices to a separate bowl and set aside.
In the same saucepan (now empty), add jalapenos, tomatillos, and onion. Add just enough water to cover the vegetables and boil for about 10 minutes, until vegetables are softened.
Strain the cooked vegetables and add them to a blender. Add cilantro and 1 cup of the reduced carnitas juices, then blend until smooth.
Next, heat vegetable oil in a medium saucepan over medium heat. Add minced garlic and cook until fragrant, about 30 seconds. Then, add the sauce from the blender and simmer for 10 minutes, until slightly thickened. Season with salt, to taste.
Serve immediately over burritos, enchiladas, or tacos, or store in the fridge for up to 5 days.