Here is a recipe I found online:
1 lb pork tenderloin
1 large yellow onion, finely chopped
1 garlic clove, minced
4 pinches salt
4 pinches black pepper
Mix the spices with your hands, squeezing the onions until they get juicy.
Place some of the spice mixture in a dish, place the meat on top and cover with the rest of the mixture.
Cover the dish and marinate for at least 3 hours or up to 6 hours. I don't recommend to marinate over night because the onions will turn bitter.
Fire up the grill and when it has its cooking temperature, remove the marinade from the meat and grill to desired doneness.
If you go with pork tenderloin, make sure you don't overcook the meat. Take it off the grill when it is still flexible when you press on it with your finger. Place it in an oven dish, cover it and let it rest in the oven at about 200 degrees for 10 minutes. This gives the juices a chance to settle and the meat will be soooo tender.
Here is the website that had the recipe for Spiessbraten