Remove the roast from the fridge and let sit at room temperature for 30 minutes. Pat the roast dry and season with salt and pepper.
With the Instant Pot set to "Saute", sear the beef on all sides for 2-3 minutes each. Remove the beef from the pot and set aside. Temporarily turn off the Instant Pot.
Add the onions to the pot and place the seared beef on top with the fat cap facing up. Add the broth, tamari, Worcestershire, garlic powder, onion powder, and herbs de Provence to the pot.
Lock the Instant Pot and set the valve to "Sealing". Cook on "High Pressure" setting for 60 minutes. Turn off the Instant Pot and let sit for 10 minutes before releasing the pressure valve.
Transfer the meat to a plate. Tent it with foil and let it rest for 15 minutes before cutting the roast into slices or shredding the roast with two forks.
French Dip Sandwiches
To make the jus, strain the broth from the Instant Pot into a heat-proof bowl. Reserve the onions for the sandwiches. Return the broth to the pot and set the Instant Pot to "Saute". In a small bowl, mix 1 tablespoon cornstarch with 1 tablespoon water to make a slurry. Pour the slurry into the pot with the broth. Stir to combine. Allow to cook for a few minutes to thicken.
Cut the baguettes into thirds to make 6 buns. Slice each bun down the middle. Stuff each bun with sliced or shredded roast beef. Top with grated cheese and reserved onions.
Place the sandwiches on a baking sheet and broil on "High" for 2-3 minutes until the cheese is melted and bubbly. Serve with jus on the side.
Notes
Store the roast beef, softened onions, and jus in an airtight container (together or separately) in the refrigerator for up to 5 days.