Step 1Heat the oven to 400 degrees. Season the tri-tip all over with 1½ teaspoons salt and ¾ teaspoon pepper, or to taste.
Step 2In a heavy Dutch oven or lidded pot, heat the oil over medium-high heat until hot. Add the meat and sear to brown on all sides, 10 to 12 minutes.
Step 3Remove the tri-tip, then add the carrots, celery and onion and cook, stirring frequently, until the vegetables are softened and beginning to caramelize, 6 to 8 minutes.
Step 4Add the red wine and continue to cook, scraping any flavoring from the bottom and sides of the pot. Continue cooking until the wine is reduced by half, 10 to 15 minutes.
Step 5While the wine is reducing, make a cornstarch slurry: Whisk the cornstarch with 2/3 cup water until the cornstarch is dissolved.
Step 6When the wine is reduced, stir in the thyme and bay leaves, then the slurry. Finally, add the meat back to the pot and take the pot off the stove.
Step 7Place a piece of parchment paper over the meat and vegetables, then cover the pot with a tight-fitting lid. Place the pot in the oven and cook until the tri-tip is fork tender, 1½ to 2½ hours, checking occasionally.
Step 8When the meat is tender, remove the tri-tip from the pot and strain the sauce, discarding the vegetables and herbs. Set the tri-tip aside to rest. The roast is best made up to this point and refrigerated for a day to give the flavors time to marry before serving.
Step 9Remove any fat that has accumulated at the top of the sauce. Slice the roast (it’s easiest to slice when cold), then reheat both, tasting and adjusting the seasoning as necessary, before serving.