In a small bowl, combine the salt, pepper, cumin, and pimentón.
Cook the tri-tip
Coat all sides of the meat with the dry rub, cover, and refrigerate for up to 3 hours.
If your tri-tip is thicker than 1 inch (2.5 cm), preheat the oven to 200°F (95°C). Place the tri-tip on a rimmed baking sheet and warm in the oven for 30 minutes to bring the internal temperature to 100°F (35°C). If your tri-tip is thinner than 1 inch, simply remove the meat from the refrigerator 45 minutes before you want to cook it and let it come to room temperature.
If your tri-tip weighs more than 1 pound (454 g), preheat the oven or increase the oven temperature to 375°F (190°C). In a large heatproof skillet, preferably cast-iron, over medium-high heat, heat the olive oil until very hot but not smoking. Add the room-temperature tri-tip and cook on all sides until well browned. Transfer the skillet to the oven and cook until the desired level of doneness is reached, about 10 minutes per pound for medium-rare. If your tri-tip weighs less than 1 pound (454 g), warm the olive oil in a large skillet, preferably cast-iron, over medium-high heat until very hot but not smoking. Add the room-temperature tri-tip and cook on all sides until desired level of doneness is reached, about 6 minutes per side for medium-rare.
Place the beef on a wire rack set over a rimmed baking sheet and let the beef rest, loosely tented with aluminum foil, for at least 10 minutes. Slice the meat against the grain and serve immediately.