Mix rub ingredients and rub all over Ribeye Roast. Place in fridge uncovered at least 1 hour or up to a day ahead.
Heat cast iron pan on low heat, melt 2 Tbsp butter and add sliced onions. Cook slowly until caramelized, then add balsamic vinegar and cook until liquid is gone. Cool completely.
Place caramelized onions, orange zest, parsley, salt and pepper in food processor and pulse until blended. Add remaining butter and pulse until smooth. Store refrigerated in airtight container until ready to use.
Take ribeye out of fridge 1 hour before roasting and set on wire rack on baking tray until it reaches room temperature.
Preheat oven to 250°F. Place ribeye in center of oven and cook until internal temperature is 110°F, about 2 - 2 ½ hours. Remove from oven and cover loosely with foil. It can rest outside of the oven for up to 2 hours.
Preheat oven to 450°F. Place ribeye back in oven and roast until internal temperature reaches 125°F, about 30 minutes. At this time take orange butter out of the refrigerator.
Let rest for at least 20 minutes before slicing. Serve with room temperature caramelized onion orange butter.