Preheat your oven to 325°F (165°C). In a mixing bowl, combine all-purpose flour, salt, black pepper, and garlic powder. Coat the cube steak evenly with this flour mixture. In a large oven-safe pot, heat vegetable oil over medium-high heat. Brown the steak on all sides until golden, about 3-4 minutes per side. Once browned, remove the steak and set it aside.

In the same pot where you browned the steak, add diced onions, celery, green bell pepper, and sliced mushrooms. Sauté over medium heat for about 5-7 minutes until the vegetables are softened and fragrant. Stir in minced garlic and tomato paste, cooking for an additional 1-2 minutes until the garlic becomes aromatic.

Add the canned diced tomatoes (with their juices), beef broth, Worcestershire sauce, dried thyme, sugar, and optional celery seed to the pot. Stir everything together, scraping the browned bits from the bottom of the pot. Let it simmer for a minute to blend the flavors.

Return the browned steak to the pot, placing it gently into the rich sauce. Cover the pot with a lid and transfer it to your preheated oven. Bake for 1½ to 2 hours until the steak is fork-tender.

If desired, mix cornstarch and water in a small bowl to create a slurry. Stir this mixture into the pot after removing it from the oven. Let it simmer on low heat until thickened, about 5 minutes.

Spoon the Swiss Steak and gravy over creamy mashed potatoes, egg noodles, or rice and enjoy!