In the crock of a 6-quart slow cooker, arrange the potatoes, carrots, and onion. Drizzle the vegetables with 2 tablespoons of olive oil and sprinkle with the Italian seasoning, 1 teaspoon salt, and ½ teaspoon black pepper. Toss to thoroughly coat.
Season the tri tip roast with thyme, rosemary, and remaining salt and pepper. Heat remaining olive oil in a large cast-iron skillet set over medium-high heat. Sear the roast for 2 minutes on all sides. Place the roast on top of the vegetables in the slow cooker.
Add the minced garlic to the skillet, then pour in ½ cup of broth and scrape the browned bits off the bottom with a wooden spoon. Stir in the remaining broth, as well as the onion soup mix and Worcestershire sauce. Bring to a simmer, then pour the contents of the skillet over the roast in the slow cooker.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours. Serve.