Preheat the oven to 275°F. Dry the roast well with paper towels and generously season it with salt and pepper.

Heat a large, ovenproof Dutch oven over medium-high heat and add the avocado oil. Sear the roast on all sides. Transfer the roast to a plate.

Reduce the heat to medium and add the chopped onion along with a pinch of salt. Cook until the onion starts to turn translucent, about 5-6 minutes. Add the garlic and cook until fragrant, about 30 seconds. Stir in the tomato paste and cook until it starts to brown, about 3-4 minutes.

Pour in 1 cup of the beef stock to deglaze the bottom of the pot, scraping up any brown bits with a spoon.

Nestle the roast in the pot. Scatter the carrots, parsnips, celery, and mushrooms around the sides of the roast. Pour in the remaining stock and top with the sprigs of thyme and rosemary.

Cover the pot and transfer it to the oven. Cook for about 3 hours, until the roast is extremely tender and the vegetables are cooked through.

Remove the roast and season the broth and vegetables with salt and pepper, to taste.