Preheat the oven to 275°F.

Chop the onion, pepper, carrots, and celery into 1/2-inch pieces. Mince the garlic.

Place the steaks on a cutting board and season both sides with the salt and black pepper.

Place the flour in a large shallow bowl. Using tongs, dredge the steaks one at a time in the flour, evenly coating both sides.

Set a large Dutch oven over medium-high heat and add the oil. Once oil is very hot and shimmering, add the steaks. Sear for 2 minutes without moving. Flip and sear the other side for 1-2 minutes longer. Transfer the seared steaks to a plate. Sear in batches if Dutch oven gets too crowded. Add more oil as needed.

If the bottom of the pot seems dry, add more olive oil. Add the onion, pepper, carrots, and celery to the pot and sauté for 5-6 minutes, until lightly browned and softened.

Add the beef broth, tomatoes, sugar, and Worcestershire sauce to the Dutch oven. Stir, scraping up any browned bits from the bottom. Return the steaks to the pot, spooning some of the vegetables over top. Cover the Dutch oven with a lid.

Bake the steaks on middle rack of oven until the meat pulls apart easily with a fork, 2-2 1/2 hours.

Check to see that Swiss steak is done. Remove from oven or add time as needed.

Place the Dutch oven carefully on the stove. Use tongs to transfer the meat from the pot to a plate.

Ladle approximately 1/4 cup of the broth from the pot into a small bowl. Add the cornstarch to the bowl of broth and whisk to make a slurry. Pour the slurry back into the pot and stir to combine. Set the heat to medium-high. Cook, stirring constantly, until the broth begins to thicken, 2-3 minutes.

Turn off the heat. Return the meat to the pot and stir with wooden spoon to coat it in the vegetable gravy.

Serve the steak with vegetable gravy over a bed of mashed potatoes, if desired.