In a small bowl, combine the softened butter, minced garlic, kosher salt, black pepper, and fresh rosemary until well mixed to create a garlic herb compound butter.
Pat the trimmed tri tip dry with paper towels. Spread the compound butter evenly over all sides of the roast. Let it rest at room temperature for 30 minutes before cooking, or season it up to 1 day in advance and refrigerate until ready to use.
Preheat the oven to 250°F. Place the tri tip on a wire rack set over a sheet pan and roast until the thickest part reaches an internal temperature of 125°F using an instant-read thermometer.
Remove the tri tip from the oven and let it rest for 20 minutes. During this time, the internal temperature will continue to rise about 5 degrees. The slow roasting process allows the fat to render gradually, while resting gives the juices time to redistribute throughout the meat for a tender, juicy roast.
Heat a large cast iron or stainless steel skillet over high heat until it is very hot and just beginning to smoke.
Add a thin layer of avocado oil, or another high smoke point oil, to the skillet.
Place the tri tip in the skillet fat cap side down and sear for about 1 minute, or until a deep brown crust forms. If it takes longer than 2 minutes to brown, the skillet is not hot enough and the meat may begin to overcook.
Turn the tri tip over and sear the second side for about 1 minute, or until nicely browned.
Transfer the tri tip to a cutting board and slice into ¼-inch thick slices, cutting against the grain for the most tender bite.