In a large slow cooker, arrange the potatoes, carrots, and onion. Drizzle the vegetables with 2 tablespoons of olive oil and sprinkle with the Italian seasoning and 1 teaspoon salt. Gently toss to coat.
Make the tri-tip rub in a small bowl. Combine the dried thyme, rosemary, salt and pepper.
Then rub it into the tri-tip roast.
Afterwards, heat 2 tablespoons of canola 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 flakes, dried parsley, celery seed, paprika, pepper and Worcestershire sauce.
First bring to a simmer, then pour over the roast in the slow cooker.
Cover and cook on low for 6 to 8 hours or on high for 2.5 hours. Remove from the crockpot when potatoes and carrots are fork tender. See notes for internal temperature suggestions.
Remove the roast, potatoes and carrots to a platter.
Optional: Make a gravy. Place a fine mesh strainer over a sauce pot. Pour the juices from the crockpot into the strainer. Discard spices leftover in the strainer. Heat the saucepan with the juices over medium high heat on the stove. Make a cornstarch slurry. Mix together 2 tablespoons of water and 2 tablespoons of cornstarch. When juices are simmering, slowly whisk in the slurry to make a gravy. Cook for 3-5 minutes until gravy is thickened.
Slice tri-tip and pour gravy over the top of the vegetables and serve.