Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Halve the baby potatoes and the brussels sprouts, then toss them on the baking sheet with 1 teaspoon of olive oil, half the sea salt, and half the black pepper.
Spread the vegetables in a single layer and roast for 15 minutes.
While vegetables roast, pat the tri-tip dry and season with the remaining salt and pepper.
Heat the remaining teaspoon of olive oil in a cast-iron skillet over medium-high heat. Sear the tri-tip for 2-3 minutes per side until a golden-brown crust forms.
Mince the garlic and finely chop the rosemary, then press them onto the top surface of the seared steak.
Move the roasted vegetables to the edges of the baking sheet and place the seared tri-tip in the center.
Return the sheet to the oven and roast for an additional 10-12 minutes, or until the steak reaches an internal temperature of 135°F for medium-rare.
Remove from the oven and let the steak rest on a cutting board for at least 5-10 minutes to lock in the juices.
Slice the tri-tip against the grain and serve alongside the crispy roasted potatoes and sprouts.