Thirty minutes before cooking, remove the meat from the refrigerator.
Preheat the oven to 300°F. Set a roasting rack in a roasting pan.
Pat the meat dry with paper towels and sprinkle on all sides with salt. Heat some oil in a large frying pan over high heat. (I know, this annoyed me too. “some oil” as an instruction in a recipe?!? But, just pour in enough to cover the bottom of the pan and you’ll be fine.)
When the oil shimmers, add the meat and sear, without moving it, for 1 to 1½ minutes to brown the bottom.
Turn the meat over, add the butter, rosemary, garlic, and lemon slices, and brown the second side of the meat, another 2 minutes or so, as it browns, tilt the pan from time to time and baste the top of the meat with the butter mixture.
Transfer the meat to the rack and arrange the lemon slices, rosemary sprig, and garlic clove on top.
Put the roasting pan in the oven and roast for 40 - 60 minutes, depending on the thickness of the roast, until the temperature in the center of the meat is about 135°F. Let the meat rest on the rack in a warm spot (such as the back of the stove) for about 30 minutes for medium-rare, allowing the juices to redistribute.
Cut the roast into thin slices: the grain in the tri-tip does not follow a straight line, so adjust the angle of the knife as you carve to continue cutting against the grain. Arrange the meat on a platter and garnish with the lemon slices, rosemary, and garlic.