Mix the brown sugar, ground chipotle, paprika, mustard powder, cumin, and salt together in a bowl.
Season the roast the day before. Place in your refrigerator overnight uncovered.
Rough chop the parsley and place into a blender. Add remainder of the chimichurri sauce ingredients and pulse for 15 seconds. Pour into serving vessel and set aside.
Heat oven to 450 degrees. Roast for 20 minutes, then turn heat to 350 degrees and continue to roast until the meat registers 115 degrees on an instant-read thermometer for rare, 125 for medium rare (it will continue to cook after you pull it out of the oven). Timing depends on your oven, your pan and the shape of your roast, so start checking after the meat has been in the oven for 30 mins.
Let meat rest at room temperature for 20 minutes before carving.
Serve roast slices with chimichurri sauce.