Preheat the oven to 325°F.
Pat the sirloin tip roast dry with paper towels. Season generously on all sides with salt, black pepper, ground rosemary, and dried thyme.
Heat the olive oil in a Dutch oven over medium-high heat. Sear the roast on all sides until well browned, about 3–4 minutes per side.
Remove the roast and set aside. Add the sliced onion to the pot and sauté for 3–4 minutes until starting to soften.
Stir in the minced garlic and cook just until fragrant.
Pour in the beef broth to deglaze the pot, scraping up any browned bits from the bottom.
Return the roast to the Dutch oven. Nestle it on top of the onions and spoon some of the broth over the meat.
Cover with the lid and roast in the oven for about 1 hour to 1 hour 15 minutes, or until the internal temperature reaches 135°F for medium roast (it will rise to ~140°F as it rests).
Remove the Dutch oven from the oven. Transfer the roast to a cutting board, tent loosely with foil, and let rest for 15 minutes before slicing.
Slice against the grain and serve warm with pan juices and onions.