Rub tri-tip roast with olive oil on both sides. Combine seasonings starting from salt - rosemary and rub onto tri-tip on both sides. Refrigerate overnight.
Remove roast from refrigerator and let sit at room temperature for 1 hour.
Preheat oven to 450 degrees F.
Place tri-tip on roasting rack. Roast for 30 minutes or until internal temperature reads 130 degrees F for medium-rare. Cook until 140 for medium. Remove from heat and loosely tent with foil. Let rest 15 minutes.
Prepare the chimichurri while tri-tip is cooking. Chop parsley, shallot, and garlic until finely minced. Mix together in a medium bowl with red pepper flakes, lemon zest, and lemon juice. Add oil and season with salt and pepper. Let sit for 30 minutes to allow flavors to meld together.
Slice tri-tip against the grain and serve with chimichurri.