Place beef roast and onions in a large resealable bag.
In a small bowl, combine the wine, oil, garlic, salt, pepper, rosemary, celery seeds, thyme leaves, and sage as a marinade.
Pour the marinade mixture over top of the beef roast and seal the bag. Refrigerate the roast for up to one day.
Preheat the grill to high heat. Remove the roast from the marinade, retaining the marinade.
Place the roast on the rotisserie spit, making sure to get it as balanced as you can. Secure it tightly and place it on grill over high heat.
After 15 minutes, reduce the heat or move the coals away from the roast and continue cooking it.
Pour 2 cups of the marinade into a saucepan. Bring it to a boil, reduce the heat to medium, and simmer for 3 to 4 minutes, stirring often. Remove it from the heat and add the butter.
Baste the roast with the basting mixture every 30 minutes during the cooking time.
Continue cooking until done, about 1 hour 30 minutes. Medium rare is best for eye round roast. Use a meat thermometer to ensure your roast is done to your liking.
Remove the roast to a platter and allow the roast to rest for 10 minutes.
Slice thin against the grain.