In a blender or small food processor blend marinade ingredients with salt and pepper to taste.
In a large sealable plastic bag combine pork with marinade. Seal bag, pressing out excess air, and put in a shallow baking dish. Marinate pork, chilled, turning occasionally, at least 1 day and up to 2 days.
Let pork stand at room temperature about 30 minutes before grilling. Remove pork from marinade, letting excess drip off, and grill on an oiled rack set 5 to 6 inches over glowing coals, turning every 5 minutes, until a meat thermometer registers 160°F., 15 to 20 minutes.
Transfer meat to a cutting board and let stand 5 minutes before slicing.
Serve pork with onion marmalade.
To make the jalapeno onion marmelade:.
In a large heavy skillet cook onions in oil with salt and pepper to taste over moderate heat, stirring, until softened. Add jalapeños and cook, stirring, 1 minute. Add honey or sugar and cook, stirring, 1 minute. Add vinegar and simmer, stirring, until almost all liquid is evaporated. Add water and simmer, stirring, until mixture is slightly thickened and onions are very tender, about 10 minutes. Season marmalade with salt and pepper. Marmalade may be made 2 days ahead and chilled, covered. Reheat marmalade before serving.