To prepare the roast, place the beef on a plate lined with paper towels.
Let the roast come to room temperature and pat the entire roast dry with more paper towels.
Season all the sides of the roast with 2 teaspoons (12 g) of the salt and the onion powder.
Warm a 12-inch (30-cm) cast-iron skillet over medium heat.
Add the avocado oil and heat it until it’s shimmering.
Carefully lay the roast in the pan, laying it down facing away from you to avoid a hot oil splatter.
Sear the roast on all sides for 3 to 4 minutes per side, or until the meat’s surface has a golden-brown sear.
While the roast sears, place the potatoes, carrots, celery, onion, garlic, rosemary, mushrooms and apple cider vinegar in an 8-quart (7. 7-L) slow cooker.
When the roast is completely seared, remove it from the skillet and transfer it to the slow cooker.
Deglaze the skillet with 1 cup (240 ml) of the bone broth, scraping the bottom of the skillet with a wooden spoon to loosen the cooked-on bits and incorporate them into the broth while being careful to keep your hands away from the steam as it will be quite hot.
Pour the bone broth over the roast and veggies.
Pour the remaining 3 cups (720 ml) of bone broth over the roast and veggies, nestling the roast into the liquid.
If needed, move a few of the veggies around—it’s okay if they are along the sides of the roast, as you want the roast to be almost completely submerged.
Place the lid on the slow cooker and set it to cook on high for 6 hours.
After 6 hours, check the roast for tenderness.
It should begin to fall apart with a fork.
If needed, place the lid on the slow cooker again and continue to cook the roast for up to 2 more hours.
The cooking time will vary a bit, depending on the marbling and thickness of the roast.
Once the roast is done, transfer it to a serving platter along with the potatoes, carrots and celery.
Strain the broth into a 3-quart (2. 9-L) saucepan, setting the onion wedges on the serving platter if desired and discarding the onion skins and the other solids.
You should have approximately 3 to 3½ cups (720 to 840 ml) of broth in the saucepan after straining.
To make the gravy, taste the mixture you strained into the saucepan.
Depending on the broth you used, you may need to add more salt, so adjust the salt level accordingly.
Add the black pepper and fresh rosemary (if using), then stir the ingredients to combine them and taste the mixture again.
Warm this mixture over medium-low heat.
While the broth mixture warms, combine the cassava flour and water in a small bowl to make a smooth slurry.
While whisking the warm broth, pour in the slurry—the mixture should begin to thicken as you whisk.
Reduce the heat to low and add the balsamic vinegar (if using).
Stir to combine the vinegar with the gravy.
Serve the pot roast and the veggies with the gravy on the side.
You can serve the mashed potatoes from our Cottage Pie with this roast.