In a large zip-top bag, combine 1/2 cup all-purpose flour, 1/2 teaspoon seasoned salt, and 1/2 teaspoon garlic powder. Add the 2 pounds beef cubed steak (about 6 pieces) one at a time and shake the bag to coat each piece evenly.
Heat 2 teaspoons vegetable or olive oil in a large nonstick pan over medium-high heat.
Add the coated cubed steak to the hot pan and brown on both sides. Work in batches if needed so the pieces are not crowded. (Brown until a golden crust forms, about 1–3 minutes per side.)
Transfer the browned cubed steak pieces to a 6-quart slow cooker as you finish browning them.
Place 1 large onion, sliced, evenly on top of the cubed steak in the slow cooker.
In a bowl or measuring cup, mix together 1 (10.25-ounce) can condensed cream of mushroom soup, 1 cup water, 1 envelope brown gravy mix, and 1 envelope onion gravy mix until smooth.
Pour the soup-and-gravy mixture evenly over the onions and cubed steak in the slow cooker.
Cover and cook on LOW for 6 to 8 hours or on HIGH for 4 to 6 hours, until the meat is tender.
Serve hot, garnished with chopped parsley if desired.