Prepare the slow cooker with nonstick cooking spray.

In a shallow dish, stir together the flour, salt, dried mustard, and black pepper. Dip the cubed steaks into the mixture, coating on both sides.

Heat a large skillet over medium heat. Add half of the oil and heat until hot.

Add half of the cubed steaks to the hot oil and brown them well on both sides.

Transfer the browned steaks to the prepared slow cooker and continue with the second batch of cubed steak, adding additional oil as needed.

When all steaks are browned and moved to the slow cooker, add the onions to the skillet and cook until softened, scraping the bottom to release any stuck-on pieces. Cook the onions for 2 to 3 minutes.

Stir in the garlic and cook just until heated through.

Transfer the onions and garlic to the slow cooker, spooning them over the cubed steaks.

In a mixing bowl, stir together the tomatoes with the Worcestershire sauce, paprika, thyme leaves, and dried oregano. Add salt and black pepper, if desired.

Pour the mixture over the steaks and onions.

Cover the slow cooker and cook on low for 6 to 8 hours.

If desired, serve the steaks with tomato gravy over rice or mashed potatoes.