Season the steaks. Mix the flour, garlic powder, and onion powder in a shallow bowl. Season the steaks with salt and pepper, then dredge them in the seasoned flour mixture. 

Pan-sear. Heat the olive oil in a large skillet. Once hot, add the steaks and sear so they brown on both sides. Set them aside on a plate.

Make the gravy. Sauté the onions in the now-empty skillet, then add the butter and wait for it to melt. Stir in the onion soup mix, then the beef broth while stirring continuously. Bring the mixture to a simmer before adding the mushrooms.

Simmer in the gravy. Return the steaks to the skillet, nestling them in the gravy. Cover with a lid and let everything simmer until the gravy thickens.

Slow cooker method: Coat and sear the steaks as normal before placing them in the slow cooker with the onions and mushrooms. Whisk the broth and onion soup mix in a bowl, then pour the mixture over the beef and veggies. Cover and cook on low for 6 to 8 hours. Add the butter in the last 30 minutes, then serve.