Build a medium-high heat fire for direct grilling.
Lather the tenderloin in oil then season with salt & pepper. Place in fridge and let seasoning set for 30 minutes.
Place tenderloin over fire and let cook until internal temperature is 117F (about 30 minutes). Make sure to rotate every 5 minutes to prevent burning. Also, baste in melted butter for the last 10 minutes for additional crust.
As steak is cooking, push to one side of the grill and preheat your cast iron. Add oil to skillet as it preheats. Once hot, add garlic and let brown for 2-3 minutes. Add onions, thyme, dijon mustard, red wine, salt and pepper. Stir occasionally and let cook till soft (about 8 minutes).
Once steak is done, pull off grill and let rest in a cooler for 12 minutes.
Once rested, top steak with onions and chopped parsley.
Slice, serve & enjoy!