The night before or the morning of the day you plan to cook this roast, sprinkle the roast all over with 1 teaspoon kosher salt. Place the roast in a dish and cover. Refrigerate until ready to use.
Remove the meat from the fridge at least 30 minutes before you are ready to cook to allow it to come to room temperature. Meanwhile mix together the seasonings (1 Tablespoon kosher salt through the cumin) in a small bowl. Remove the meat from the dish and pat dry with paper towels. Liberally sprinkle the seasoning mix over the roast and rub into the meat.
Preheat your oven to 250 degrees. Heat a large cast iron skillet over high heat and add the vegetable oil to the pan. Sear the roast on both sides until a nice brown crust forms, about 3 minutes per side. Place the roast on a rack-lined baking sheet and into the preheated oven. Cook until the internal temperature of the roast reaches about 115-120 degrees (invest in a digital thermometer!), this will take about 30-40 minutes for a flat sirloin roast and about an hour for a thicker roast. Be patient. Once the roast has reached 115 degrees, turn off the oven and leave the roast in the warm oven for 30 minutes or until the internal temperature reaches 140 – 145 degrees for medium rare (leave it longer if you like but don’t cook the death out of it). Do not open the door as you will let out all of the hot air.
While the roast is cooking, mix together the remaining ingredients for the parsley and tomato sauce. It should have a nice vinegary bite, but not too much so adjust the oil and vinegar to your taste.
Remove the roast to a carving board and slice thinly, across the grain, and top with the parsley sauce.