Preheat your oven to 325°F (165°C). In a shallow dish, coat the cube steaks generously with a mix of all-purpose flour, salt, pepper, and garlic powder. Heat ¼ cup of vegetable oil in a large oven-safe pot over medium-high heat. Once the oil shimmers, add the steaks and brown them for about 4-5 minutes on each side until they develop a golden crust. Remove the steaks and set them aside on a plate.

In the same pot, reduce the heat to medium and add the diced onion, celery, and sliced green bell pepper. Sauté these vegetables for about 5-7 minutes until they are tender and the onion turns translucent. Next, add the sliced mushrooms and continue cooking for 2-3 minutes until they soften. Stir in the minced garlic and tomato paste, cooking for an additional 1-2 minutes until fragrant and slightly caramelized.

With the vegetables now softened, pour in the diced tomatoes, beef broth, Worcestershire sauce, dried thyme, sugar, and a pinch of celery seed. Stir well and make sure to scrape any browned bits from the bottom of the pot, as these add rich flavor to the gravy. Bring this mixture to a gentle simmer.

Carefully return the browned steak to the pot, ensuring it is fully submerged in the savory sauce. Cover the pot with a lid and place it in the preheated oven. Let it bake for 1½ to 2 hours, checking periodically until the steak becomes fork-tender.

If you prefer a thicker gravy, mix 2 tablespoons of cornstarch with an equal amount of water in a small bowl. Stir this mixture into the pot after the simmering time and return it to the stovetop. Cook over medium heat for another few minutes, stirring until the gravy thickens to your desired consistency.

Once ready, gently transfer the steak and sauce to a serving platter or plate the dish directly over creamy mashed potatoes, fluffy rice, or buttered noodles. Garnish with fresh parsley if desired.